GLX Pbuffers are not bindable so implementing bindable pbuffers would
have to be one of the following:
 - Implement Pbuffer as GLXPixmaps, with complication arising in the
   initialization.
 - Implement Pbuffers as GL textures, which would require a large change
   of the EGL implementation.
 - Fake bindable Pbuffers with glCopyTexImage2D, with obvious
   performance problems.

BUG=angleproject:892

Change-Id: I7d9cd5a4aa2d35343244be3cbe55fc1e492f6928
Reviewed-on: https://chromium-review.googlesource.com/273056Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>