Add ability to combine multiple textures and get a new texture

[bacable]

I'd like to not have to manually generate all textures and save them as pngs before the game launches, especially when it comes to making card or tile textures. I'd like to just be able to provide a few images (that might be part of an existing spritesheet) and then get the new texture from it.

Maybe generate new sprite atlases out of those composite images as well, to get the performance benefit?