This implements a new overlay that blends the bakground color over the
objects that are not in the same mode as the active object, making
them fade with the background.
This is especially needed for sculpt mode as there is no other overlay
or indication in the viewport to display which object is active.
This is intended to be used with D7510 in order to have a faster
workflow when sculpting models with multiple objects.
Reviewed By: fclem
Differential Revision: https://developer.blender.org/D8679
