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This CL enables alpha blending at scanout for the primary plane.
In this way chromecast will be able to punch a hole in the primary
plane and show content underneath.

Note that on platform using legacy pageflip we'll still be
scanning out without alpha blending (crrev.com/c/964984).

Bug: 821944, b/74997524
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel
Change-Id: I654935dc630f7a9f6cb4717cd574a21a2749c442
Reviewed-on: https://chromium-review.googlesource.com/976961Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Commit-Queue: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545478}