Combining submeshes atlases sprites

Did you ever try to import an OBJ that uses multiple textures? Did you ever want to combine each of the subimages into a single vertex buffer, but doing so smooshes all of the UVs on top of each other and makes texture mapping break? Now combining submeshes will stick all of the textures into a single atlas, and remap them accordingly.

Note: any of the submeshes rely on texture repetition it’ll probably be an even bigger disaster than before.

I also changed the Y-up/Z-up button so that it rotates the result automatically.

Might have fixed a few bugs or something too. I’m not sure, because it’s been a long time since I looked at this.