Implement FramebufferGL. BUG=angleproject:885 Change-Id: Ifb5818f185236c671cd7f20ed352edb887b49675 Reviewed-on: https://chromium-review.googlesource.com/258420 Reviewed-by: Kenneth Russell <kbr@chromium.org> Reviewed-by: Brandon Jones <bajones@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>