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
