CGL: implement EGL_ANGLE_wait_until_work_scheduled It is as important to wait until scheduled in CGL, as it is in Metal. CGL waits until scheduled when glFlush() is called. Fixed: angleproject:8112 Change-Id: Id4a9e87804c6df1828b35cfd30c8427314820e52 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4386400 Commit-Queue: Kenneth Russell <kbr@chromium.org> Auto-Submit: Kimmo Kinnunen <kkinnunen@apple.com> Reviewed-by: Kenneth Russell <kbr@chromium.org>