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Fix T82155: Fade inactive geometry overlay not working with instanced geometry

This enables the overlay for instanced geometry.

After this change, objects that are an instance of the current active
object (which are also being modified in the current active mode) won't
fade, which is different from the previous behavior.

Reviewed By: fclem

Maniphest Tasks: T82155

Differential Revision: https://developer.blender.org/D9362