Composite sprites

[Schmendrick]

It would be nice to have support for defining sprites in terms of other sprites. This would allow for some saved memory for sprite sheets, but would also allow more elegant generic mod behavior.

For example, the Liquid Station mod could be easily written for any liquid in the game by forming the recipe and filled barrel icons from the empty barrel icon and liquid icons, or any of the inserter variants could be formed by an icon of the base inserter, and one of a few smaller symbols added by the mod itself.

Some form of a tint modifier or code to let mods modify graphics in the same way as the (rumored/proposed) [multi]player coloring might be nice, too.