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.