TNT: make tnt:boom visual a particle, and larger

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.

init.lua

@@ -309,14 +309,24 @@ function tnt.boom(pos, def)
 end
 
 minetest.register_node("tnt:boom", {
-	drawtype = "plantlike",
+	drawtype = "airlike",
-	tiles = {"tnt_boom.png"},
 	light_source = default.LIGHT_MAX,
 	walkable = false,
 	drop = "",
 	groups = {dig_immediate = 3},
 	on_construct = function(pos)
-		minetest.get_node_timer(pos):start(0.5)
+		minetest.add_particle({
+			pos = pos,
+			velocity = vector.new(),
+			acceleration = vector.new(),
+			expirationtime = 0.4,
+			size = 30,
+			collisiondetection = false,
+			vertical = false,
+			texture = "tnt_boom.png",
+			playername = nil,
+		})
+		minetest.get_node_timer(pos):start(0.4)
 	end,
 	on_timer = function(pos, elapsed)
 		minetest.remove_node(pos)