backend-drm: Allow for views to reach overlays/underlays planes

backend-drm: Allow for views to reach overlays/underlays planes

In this manner we will allow views to reach the overlay (or underlays)
even if the damage tracking will detect that the new view will
occlude the view underneath it.

Renames occluded_region to planes_region, and uses occluded_region
to represent the region where we add each view's visible-and-opaque region.
Sprinkle some comments about each region.

Re-uses the view's clipped region to determine visible-and-opaque region
which is accumulated (for both renderer and HW planes cases) into
occluded_region. The current view's clipped_region is then checked against
occluded_region.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Suggested-by: Daniel Stone <daniel.stone@collabora.com>