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In preparation for hardware plane underlay support in ChromeOS, we
need to allow for RGBA primary framebuffers. When using an underlay,
we cut out a transparent rectangle in the primary framebuffer to allow
the underlay to show through, while still allowing for RGBA content on
top of the underlay (for example, video controls or annotations).
When not using an underlay, the primary framebuffer will be displayed
as RGBX (opaque) to avoid unnecessary blending in the display
controller.

Bug: 789288
Change-Id: I2fc94e524e250ec9b7e11cd6801e1c8308046a10
Reviewed-on: https://chromium-review.googlesource.com/801974Reviewed-by: Alex Sakhartchouk <alexst@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Reviewed-by: David Reveman <reveman@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: Kristian H. Kristensen <hoegsberg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542877}