Optimization: Exit early when resizing material slots.

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

source/blender/blenkernel/intern/material.c

@@ -900,9 +900,15 @@ void BKE_objects_materials_test_all(Main *bmain, ID *id)
   }
 
   BKE_main_lock(bmain);
+  int processed_objects = 0;
   for (ob = bmain->objects.first; ob; ob = ob->id.next) {
     if (ob->data == id) {
       BKE_object_material_resize(bmain, ob, *totcol, false);
+      processed_objects++;
+      BLI_assert(processed_objects <= id->us && processed_objects > 0);
+      if (processed_objects == id->us) {
+        break;
+      }
     }
   }
   BKE_main_unlock(bmain);