Expose entity graphics within LuaEntityPrototype

[BurninSun]

I want to use rendering.draw_animation or rendering.draw_sprite to draw my own copies of entities. Currently, the only way to get those graphics is by iterating entities and copying their graphic information into an animation/sprite prototype during data stage.

I would like LuaEntityPrototype to have an attribute of sprite or animation which returns something which can be passed directly to rendering.draw_animation/sprite.