Allow the on_blast API to pass an itemstack.

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.

mods/tnt/init.lua

@@ -93,7 +93,12 @@ local function destroy(drops, pos, cid)
 	end
 	local def = cid_data[cid]
 	if def and def.on_blast then
-		def.on_blast(vector.new(pos), 1)
+		local node_drops = def.on_blast(vector.new(pos), 1)
+		if node_drops then
+			for _, item in ipairs(node_drops) do
+				add_drop(drops, item)
+			end
+		end
 		return
 	end
 	if def and def.flammable then