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Edit Mesh: multi-thread auto-smooth & custom normal calculations

Supported multi-threading for bm_mesh_loops_calc_normals.

This is done by operating on vertex-loops instead of face-loops.

Single threaded operation still loops over faces since iterating
over vertices adds some overhead in the case of custom-normals
as the order used for accessing loops must be the same as iterating
of a faces loops.

From isolated timing tests of bm_mesh_loops_calc_normals on high
poly models, this gives between 3.5x to 10x speedup,
with larger gains for meshes with custom-normals.

NOTE: this is part one of two patches for multi-threaded auto-smooth,
tagging edges as sharp is still single threaded.

Reviewed By: mont29

Ref D11928