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Subtract damage caused by unchanged UI elements on top of HW underlay.
This optimization saves recomposting quads that haven't changed.
In a steady state when only the underlay contents are changing, we'll be
able to completely skip GPU compositing in many cases.

Bug: 795008
Test: Unit Test-> UnderlayTest.DamageSubtractedWhenElementsOnHwUnderlayNotChanged
Change-Id: I9ae1b9c50f63a7a8e50e3f9e16f6ad58adc5308c
Reviewed-on: https://chromium-review.googlesource.com/c/1461102Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Maggie Chen <magchen@chromium.org>
Commit-Queue: Mark Yacoub <markyacoub@google.com>
Cr-Commit-Position: refs/heads/master@{#636213}