This multithreads the main loop of unsubdivide grids extraction from the
original mesh. On a Threadripper 3990X, this loop goes from 0.369013 to
0.017537 when unsubdividing suzane with 6 subdivisions applied (2M
The entire operation still takes around 16 seconds as the slowest part
is dissolving the vertices to generate the base mesh (it takes almost 11
out of 16 seconds), so the speedup in the unsubdivide and rebuilds
subdivision operator is not that noticeable.