Previous code was using BKE_mesh_uv_vert_map_get_vert to somewhat
detect identical UV values on mesh vertices. But this was not
handling the case when separate mesh vertices still use the same UV
values (e.g. cube with all six faces mapped to whole image).
Replace usage of BKE_mesh_uv_vert_map_get_vert with a simple "unique UV
value" map, very similar to how OBJ normals are calculated for export.
Measurements on a somewhat extreme case: exporting "Rungholt" Minecraft
level from https://casual-effects.com/data time (Win10, Ryzen 5950X)
goes 2.9sec -> 2.2sec, and resulting file size 486MB -> 231MB. This
particular model has a lot of mesh faces mapping to the same places
in UV map.
On less extreme cases the timings are similar between old and new
code, with a tiny speedup in most tests I've tried.
05a63e3705