This uses a vmanip to count adjacent tnt nodes and explodes them
all at once, using an inverse square law to recalculate the radius.
The maximum explosion becomes 125 nodes of tnt yielding a radius of
15 nodes, which does not break my machine and makes it return
in under a second.

This makes both bigger explosions and less stability issues.

The drop code has been simplified and now drops at all times a
reasonable amount of drops, never blanketing the area with drops,
even at the larges explosion level.

Particles are scaled up according to explosion size as well - a
bigger explosion will show bigger particles.

To scale the tnt:boom particle, we move it to the _effects() function.

Introduces an `on_blast(luaobj, damage)` callback that mods can attach
to an entity def. The function will get called with the damage that
TNT would make.

The function should return three values:

  bool do_damage, bool do_knockback, table drops

do_damage allows the mod to tell the TNT code to perform damage on
the entity for the mod. The mod code should not do anything with
the entity HP. The entity should not be immortal. If false, then
the entity will not be damaged by the TNT mod.

do_knockback allows the mod to tell the TNT mod to perform an
entity knockback effect. If false, no knockback effect is applied
to the entity.

the drops table is a list of items to drop. It may be nil. E.g. {
"wool:red" }.

I've documented both on_blast() API methods in game_api.txt. It is
a better place than lua_api.txt.

Any second explosion near a first TNT explosion will punch all
entities found nearby, including item drops. This causes the
item pickup code to think the item was picked up, but by
a `nil` player, thus removing the item.

We query for the immortal entity group, and if the item is in
the immortal group, do not punch the item.

We reuse the tnt:boom texture for a particle that is added by the
on_construct() of tnt:boom, and has a short expiry time (0.2sec).
It is 3 nodes larged, centered on the explosion.

We then make tnt:boom airlike so it doesn't have a texture, and it's
the thing that emits lots of light (we could even make it exist a
bit longer).

The nice thing about particles is that the client is less susceptible
to lag and will always remove them as fast as possible, so this makes
the visual more constant and responsive.

The effect is similar, and the reduction in particles is a small
boost in responsiveness.

To compensate, I've lowered the spawner time and expiration length
as well.

This allows TNT to be used for e.g. traps (drop it from the
ceiling) or weird tnt effects with other explosions.

We add a +/- 0.5 random value to the velocity vector of
ejecting nodes.

I've spotted a lot of nodes going exactly straight up if blowing
up sand above TNT. The extra variation looks less artificial.

We apply punch damage to mobs caught in the blast radius, as
this code previously only hurt players.

We "move" players back 1 node if they're caught in the blast, and
slightly up. We can't "eject" players due to missing API code to
support that, unfortunately.

Adds a minor helper function that allows efficient retrieval of
several inventories from a node inventory. We use this helper to
quickly retrieve the items in chests, vessel shelves, book shelves
and furnaces, and return these with the nodes itself to the TNT caller.

The TNT caller then performs the entity physics, and we don't need
to do anything else.

We disable TNT doing anything with bones.

We expose a bug in the code that drops the items - metadata was lost
entirely. This patch corrects that by properly copying the metadata
and creating the drops list inclusive metadata.

This will make sand and gravel blocks on top of TNT actually fly
in the air.

  https://youtu.be/4omndVZijLc

Let's just call it "terminal" velocity.

We add on_create() handlers for both burning TNT and burning
gunpowder. Because gunpowder will explode TNT in 1 second,
and not 4, we need to modify the 4 second timer after we
make the TNT burning. Other mods can now place burning TNT
that will by default explode after 4 seconds.

I spotted two places where under stress (many explosions) luajit would
end up passing nil to these functions. I'm not entirely sure how,
but it seems good form to guard against it, which does make it
more robust. After this patch, I'm not able to crash the server. With
many explosions, it may still lag significantly, but always returns
in the end.

We define the blast intensity as the square of the tnt_radius, divided
by the square of the distance to the explosion center, where distance
is limited to 1 at the lower end.

When destroying nodes, we calculate the intensity for each node, and
only destroy the nodes when the intensity is 1.0 or larger. To avoid
perfectly spherical explosions, we make sure to retain a randomness
factor of 20%. This will make explosion edges jagged and not smooth,
but not too much.

We pass the calculated intensity to on_blast() functions as well,
except we take the jitter here out and make sure it's always 1.0
or larger.

We apply a log scale to the size of the stacks ejected, so that
in larger explosions we are getting larger stacks. For normal r=3
explosions, this gives stack sizes ~6-7 or so, but for r=10 explosions
it could end up giving stacks of 25+.V

With the new high res timer, we can make TNT much better create chain
reactions. This does exactly that.

The drops list already has quantities, so let's just select the one
with the highest quantity from it, and use that as tile. Fallback
tile will therefore only be used if explosion happens in air. Oh well.

- Fix spacing errors around = and ..
- Indentation fixes
- Break up long lines

Currently the TNT mod uses a dummy vaule of 1 for this.

We add a dirt-like particle (drawn from scratch, uses some
colors from default_dirt's palette) to spawn many particles
that have collision enabled around the center of the blast.

This has the effect of obscuring the center of the blast, as
that is a painfully visible empty area when the explosion happens,
as there's only a little spark.

The dirt particles bounce around the walls and floor a bit,
and disappear rapidly, well before the smoke particles disappear.

This is a nice visual distraction that obscures the sudden
appearance of the gaping hole, and makes it a whole lot more
believable.

But not too much.

TNT is a bit underwhelming at the moment. We can make it a bit
more interesting by ejecting not just one or two itemstacks,
but a bunch of them. This code splits up the drops into
separate itemstacks that are 2-5 items together, which
results in generally roughly 10 itemstacks being ejected.

Since now we have multiple ejecta, it makes sense to tune
the ejecta velocities a bit better to get the appearance of
an actual explosion better. The items will not all start
with the same vertical velocity, since that would look
like fireworks. Instead we give them all a different vertical
speed.

If the node is special and has an on_blast() handler, we need
to call it instead of getting node drops manually. However, we
do want to know if drops should be added for the special nodes,
so we modify the on_blast() handler code to allow the nodedef
handlers to pass back itemstacks. This could be used by e.g.
the doors mod to drop door items after a blast.

Since this API is documented in lua_api.txt, a separate PR will
be incoming to update the on_blast() documentation.

By adding the timer to the tnt:tnt_burning node it will help mods add the block and cause an explosion after 4 seconds instead of doing nothing like in it's current state.

This option is not documented nor is it used with any other mod in Minetest Game so why should it be used in TNT?

Spread ABM intervals evenly across 1 to 16 seconds
16s ensures no nodes are missed when player walks past
Adjust chance values to compensate, for identical action rates
Combine lavacooling ABMs into one, return to chance = 1
Grass growth: add 'neighbors = "air"' to avoid
processing the thousands of underground dirt nodes
Grass death: Reduce action rate to that of grass growth
Fire: Use chance = 1 for flame extinguishing
and flame removal when mod is disabled

Remove line if set to the default of 'true'

Uses group “connect_to_raillike”

Copy pos before giving it to on_blast because it can modify it; also remove a debug print that had nothing to do here

Implemented nodes:
- Steel Door: Ignores explosion
- Locked Chest: Ignores explosion
- Fire: Ignores explosion
- TNT: Starts burning
- Burning TNT: Explodes immediately
- Gunpowder: Starts burning
- Burning Gunpowder: Ignores explosion