Previously this relied on the dependency graph to detect changes in the screen
datablock, which would then notify the renderers. This was rather indirect an
not even really by design. Instead use notifiers to tag specific 3D viewports
to be updated.

Includes changes to BKE_scene_get_depsgraph to accept a const Scene pointer.

Testing if this works correctly requires adding back commits 81d444c and 088904d,
since those have been temporarily reverted.