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 the first part of two for multi-threading
auto-smooth, tagging edges as sharp is still
single threaded (coming next).
Notes:
- This change is limited to multi-threading existing logic. Some other optimizations may be worth looking into but I would rather do them separately to this patch as it makes the patch harder to review and complicates debugging any regressions they could cause.
- For faster overall performance, tagging sharp edges could be multi-threaded as well - perhaps making it part of the bm_mesh_loops_calc_normals to avoid a separate blocking loop over all edges.
- Optimizations have been split into a separate patch (which requires this one to be applied first), see: D11970: Edit Mesh: multi-thread auto-smooth sharp-edge calculation.
Other changes made as part of this patch: