Combining submeshes(/materials) can now combine textures

A small change: using the “combine submeshes” button will now also combine the textures so that using this to fuse a model that uses a bunch of different textures into one piece no longer turns everything into a jumbled mess.

There’s also a setting in Preferences to force the output texture to a power of 2 in size. Right now it defaults to Off until I get around to finding an algorithm that’s a little more space-efficient.