GL: Skip redundant flushes. Skip flushes when there is no work submitted since the last flush. Clone of geofflang's: https://chromium-review.googlesource.com/2911879 Bug: chromium:1181068 Change-Id: Idd8a1c61b814d746bf1e0225cfc4ca375a7bb224 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2915675 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>