Blender Git Loki

Fix T73871: improve assignement of material to selection in multi object
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This was already supported in "Select" & "Deselect" but not in "Assign".
So similar to rB6b39dc7672eb, we now check if the material corresponding
to the currently selected material slot is found in other objects
materials and assign this (instead of always assigning their 'own'
actcol).

Maniphest Tasks: T73871

Differential Revision: https://developer.blender.org/D6869