Reland: GL: Skip redundant flushes. Skip flushes when there is no work submitted since the last flush. Bug: chromium:1181068 Change-Id: Iab31f8949f7eb07dffb7b0930069cbb3ca69ef57 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2956453 Reviewed-by: Kenneth Russell <kbr@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>