TNT: Allow mods to override entity physics.

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.

mods/tnt/init.lua

@@ -138,7 +138,7 @@ local function calc_velocity(pos1, pos2, old_vel, power)
 	return vel
 end
 
-local function entity_physics(pos, radius)
+local function entity_physics(pos, radius, drops)
 	local objs = minetest.get_objects_inside_radius(pos, radius)
 	for _, obj in pairs(objs) do
 		local obj_pos = obj:getpos()
@@ -157,14 +157,31 @@ local function entity_physics(pos, radius)
 
 			obj:set_hp(obj:get_hp() - damage)
 		else
-			local obj_vel = obj:getvelocity()
-			obj:setvelocity(calc_velocity(pos, obj_pos,
-					obj_vel, radius * 10))
-			if not obj:get_armor_groups().immortal then
-				obj:punch(obj, 1.0, {
-					full_punch_interval = 1.0,
-					damage_groups = {fleshy = damage},
-				}, nil)
+			local do_damage = true
+			local do_knockback = true
+			local entity_drops = {}
+			local luaobj = obj:get_luaentity()
+			local objdef = minetest.registered_entities[luaobj.name]
+
+			if objdef and objdef.on_blast then
+				do_damage, do_knockback, entity_drops = objdef.on_blast(luaobj, damage)
+			end
+
+			if do_knockback then
+				local obj_vel = obj:getvelocity()
+				obj:setvelocity(calc_velocity(pos, obj_pos,
+						obj_vel, radius * 10))
+			end
+			if do_damage then
+				if not obj:get_armor_groups().immortal then
+					obj:punch(obj, 1.0, {
+						full_punch_interval = 1.0,
+						damage_groups = {fleshy = damage},
+					}, nil)
+				end
+			end
+			for _, item in ipairs(entity_drops) do
+				add_drop(drops, item)
 			end
 		end
 	end
@@ -303,7 +320,9 @@ function tnt.boom(pos, def)
 	minetest.set_node(pos, {name = "tnt:boom"})
 	local drops = tnt_explode(pos, def.radius, def.ignore_protection,
 			def.ignore_on_blast)
-	entity_physics(pos, def.damage_radius)
+	-- append entity drops
+	entity_physics(pos, def.damage_radius, drops)
+
 	if not def.disable_drops then
 		eject_drops(drops, pos, def.radius)
 	end