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.

Derived from Casimir's Voxelgarden texture

Tweaked the beds.save_spawns() function to compile list then output
to file with shortened co-ordinates containing 1 decimal place

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.

Similar to before, add symmetry and slightly more rounded off

Flammable nodes burn away quicker

Remove search for 'group:flora_block'
Enable flora spread on 'dirt with dry grass'

This works much more efficiently to replace old style doors.

There was a small leg wobble in the sitting animation (visible
when sitting on the boat). This removes it.

Tidy up position numbers and code
Check for flora group aswell
Improve node light check for mushroom growth
Shrooms grow in 3 and below light

This changes how dirt blocks turn to dirt_with -grass, -dry_grass
or -snow.

Previously, dirt that was sunlit would turn to dirt_with_grass no
matter what, but this happened without any context, so you could
get green patches of dirt_with_grass in the middle of a savannah or
even desert.

Dirt no longer turns to covered dirt unless it's within 1 node from
another dirt_with_grass or dirt_with_dry_grass or dirt_with_snow.
This makes dirt_with_grass "growback" a lot slower, since it now only
happens on the edges, but it retains the context nicely now.

If there is any dirt with a grass or dry grass plant, or snow on top,
and enough light, we'll convert it sporadically to dirt_with_grass
or dirt_with_dry_grass or dirt_with_snow.

This allows us to plant grass of our choice in a large dirt patch,
or in a region where otherwise that type of grass is not present.

This used to be done by 2 abms, but I've combined them in to a single
ABM that is ordered to run with maximum efficiency, solving for the
most common outcome first before attempting more complex checks.

This is technically "dirt with grass" that's just under a snow
cover, so in darkness the grass on these nodes will also die,
turning it into dirt.

This doesn't convert dirt_with_snow under snow.

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