When assigning a huge number of materials, like when importing thousands of
objects, the function `BKE_objects_materials_test_all` uses quite a lot of
resources because of the way it loops through all objects to resize the
mat-array.
By counting the amount of processed objects and comparing to the number
of users of the obdata ID, we can exit early and avoid looping through
all objects every time.
Reviewed By: mont29
Differential Revision: https://developer.blender.org/D15740
609171d8b7