Improve multires performance.

Added a new api function to stich multires grids
on specific faces in a mesh,
subdiv_ccg_average_faces_boundaries_and_corners,
and changed multires normal calc to use it.

VTune profiling showed that this was a major
performance hit once you get above 10,000 or so
base mesh faces and/or have a high number of
subdivision levels.

Here's a video comparing the difference. Note the
bpy.app_debug switch is not in the final commit.

{F10145323}

And the .blend file:

{F10145346}

Reviewed By: Sergey Sharybin (sergey)

Differential Revision:
https://developer.blender.org/D11334

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