Since we only care about driver state tracking logic, blit is unrelated
here. This further reduce the noise of test result. Instead of blit and
swap every N frame, we will issue glFlush for offscreen rendering. The
onscreen will still issue swap every frame.

Bug: b/186881553
Change-Id: Id048eaafa6dcef26d851fd19f069331d240e1bf5
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2846527
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Ian Elliott <ianelliott@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>