This was a regression of 2.4.0-alpha1 wrt. 2.3.3, where handling of
multiple volumes was refactored.
As a result, handling multi-part objects with clip_multipart_objects
disabled was broken.
With clip_multipart_objects disabled, the multi-part objects no more
clip one another and it is sole responsibility of the user to handle
overlaps.
Fixes "add part" error - PrusaSlicer 2.4.0 b2 #7366