Use scanout overlays for pepper2d when possible.

If GL image textures are supported, and we can find the image texture
target for the desired format, then use TexStorage2DImageCHROMIUM to
allocate the texture so that it may be used for scanout.

R=piman@chromium.org

Bug: 786140
Change-Id: I36875f82796614f4d5c7290f27bd0bacfce510fb
Reviewed-on: https://chromium-review.googlesource.com/779883


Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518269}