Summary:
Sort a mesh's vertices in alphabetical order, and resort the face coords accordingly. Textured meshes are not supported yet, but will be added down the stack.

This, togehter with mesh equality, can be used to compare two meshes in a way invariant to vertex permutations, as shown in the unit tests.

We do not want the submeshing mechanism to guarantee any particular vertex order, leaving that up to the implementation, so we need this function for testing.

Reviewed By: bottler

Differential Revision: D35440656

fbshipit-source-id: 5a4dd921fdb00625a33da08b5fea79e20ac6402c