The CGL pbuffers have been deprecated since OSX 10.7 but our testing
requires that an EGL implementation supports pbuffers. This commit
implement pbuffers on top of renderbuffer using only OpenGL.

BUG=angleproject:891

Change-Id: Ie8e09c9300a00800600c3a77044ba05205f805da
Reviewed-on: https://chromium-review.googlesource.com/299442Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>