Support EXT_multisample_compatibility in the GL backend
BUG=angleproject:1377
Change-Id: Ie14aceca8e01f1cbc93fd5bd06d986336fb752b3
Reviewed-on: https://chromium-review.googlesource.com/343501
Reviewed-by: Sami Väisänen <svaisanen@nvidia.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Sami Väisänen <svaisanen@nvidia.com>
