The new OBJ/MTL importer was creating a new image for any referenced
texture, even if another material (or another property of the same
material) already referenced the same texture. Make it use
BKE_image_load_exists function just like Collada or USD importers do.
Fixes T100075. Extended test coverage to count imported images;
without the fix import_cubes_with_textures_rel would have incorrectly
created 5 images instead of 4.
bea5281919