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When GLRenderer reads back from the framebuffer with glCopyTexImage2d,
it tries to be smart about what internal format to use for the new texture.

Unfortunately on CrOS it ends up using GL_BGRA_EXT, that seems to hit
a particularly slow path in gles2_cmd_decoder.

This CL changes it to RGB/RGBA, decreasing the CPU cost of CopyTexImage2d
of two orders of magnitude when applying fullscreen backdrop effects.

Bug: 900046

Change-Id: I19bd8a9d008109e336f0fd346df130c2d7878154
Reviewed-on: https://chromium-review.googlesource.com/c/1306653Reviewed-by: Kristian H. Kristensen <hoegsberg@chromium.org>
Commit-Queue: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604062}