These updates are used to recalculate normals and average values between
grid boundaries during multires sculpting. In main the affected faces
are passed as an array of pointers. Using an `IndexMask` instead reduces
memory usage by 4x per affected face (8 byte pointer to 2 byte integer),
simplifies iteration and threading, and can also improve performance.
Finding which faces are affected is now multithreaded, with its runtime
changing from 0.63 ms to 0.12 ms in a simple test sculpting on a portion
of a 1 million face grid.
Also switch to VectorSet instead of GHash for finding affected adjacent
elements. That's a friendlier data structure that probably has better
performance as well.
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