As part of the previous fix (D15410), the importer got code to track
min & max vertex indices used as part of the mesh faces. However, if
faces refer to a "sparse" (i.e. non-contiguous) subset of all vertices,
then the imported mesh would contain all the vertices between min & max
range.
Replace that with proper tracking of actually used vertex indices
for each imported mesh. Fixes T100302.
This does affect import performance a tiny bit, e.g. importing Blender
3.0 splash scene goes 21.7s -> 22.1s, and importing rungholt.obj
goes 2.37s -> 2.48s.
Importer related tests have a bunch of vertex changes in them, since
now vertices are added in the order that the faces are referring
to them. Which incidentally matches the order that the Python based
importer was creating them too.
d76583cb4a