This more closely matches the OpenGL "Pixel Buffer" concept.

Bug: angleproject:2318
Change-Id: I930b24f68cde2995488737908aae469c86b74092
Reviewed-on: https://chromium-review.googlesource.com/1012456Reviewed-by: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>