Old python exporter in 3.0 and earlier ordered faces by material,
but the new C++ exporter in 3.1+ did not, and was just writing them
in whatever is the order of the mesh data structure.
This mostly does not cause problems, except in some apps e.g.
Procreate -- for large enough meshes, this lack of
"order by material" (which ends up having more usemtl lines)
ends up creating more mesh subsets than necessary inside Procreate.
The change is not computationally heavy, e.g. exporting 6-level
subdivided Monkey mesh goes 1085ms -> 1105ms on my machine.
Reviewed By: @howardt
Differential Revision: https://developer.blender.org/D14368
eb1755be35