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Due to existing limitations in damage calculations some overlay
candidate quads will still lack a valid |surface_damage_index|. This
means that separating out the damage of a promoted overlay quad from the
damage on the primary plan is now virtually impossible. However, a
simple optimization is still possible if the overlay candidate is opaque
and sits above the primary plane (pure overlay not underlay). So in this
case the |display_rect| of the promoted overlay can be subtracted from
the primary plane damage.

The motivation for these changes is primarily improved Lacros power
performance. The special case mentioned above is far more common in
existing lacros due to the entire lacros window being contained in one
compositor frame.

Finally, it should be noted that with this change some pages now move
closer to power equivalence between Lacros and ash chrome. Specifically
the Google Keeps drawing application has now only ~6% greater power
usage on Lacros than ash chrome.



Change-Id: I83af491adb3dd2832f599d34e5849fd518ffdfa5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2634926Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Commit-Queue: Peter McNeeley <petermcneeley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845294}