Port / move the 3D/2D mapping computation code

[thewtex]

Port / move the 3D/2D mapping computation code

[marcniethammer]

This is the code that generates the mapping from the atlas mesh to 2D. It only needs to be computed once. Porting this code would simply be to be able to support different atlases going forward. (I.e., if we change the atlas we need to recompute the mapping.)