Similar to recent OBJ UV values merging commit (05a63e3705) - build
mapping of (vertex, UV) by going over the face loops directly, instead
of using BKE_mesh_uv_vert_map_get_vert and then having an additional
map on top. Provide the mapping into ready to use flat arrays, instead
of building a map and then building arrays out of that in a separate
pass.
While at it, avoid the extra cost of building all this complicated
mapping when we don't have or are not exporting UVs.
Timing tests on exporting several models into binary PLY file
(Win10, Ryzen 5950X):
- Suzanne subdivided to level 6 (2.1M verts): 0.93s -> 0.68s
- Rungholt Minecraft level (9.7M verts): 3.3s -> 2.3s
- Stanford Lucy 3D scan (14.0M verts, no UVs): 5.2s -> 1.5s
7b94e86dd9