Fix multiple texture support for animated meshnodes



This will allow 3d entity models with multiple UV textures and materials to be updated through Lua.
Currently only the first material texture gets updated which was clearly not the author's intention.

This was previously merged then (imo) incorrectly reverted. Please read the updated forum topic for full details.

Test mod and model available for download.
Forum topic: http://forum.minetest.net/viewtopic.php?pid=91372