Virtual sprites: "patch" images to fix problematic sprites

[michaeldgg2]

Some sprites have issues and fixing them usually requires modifying the entire sprite and then shipping it in mod data.

Preferable solution would be to create just "patch" images: images that, when "applied" to original sprite, should result in desired (i.e. fixed) image.

One example, where this could be used is a small area of incorrect pixels in sprite with undamaged Civ Building 2/3:

#514