We can mutate the BitSetIterator as it clears dirty bits. This removes
the risk of doing a double state update. Improves the proformance of
the GL back-end state update.

Also do an early-out before calling syncDrawArraysState.

Bug: angleproject:2763
Change-Id: Idd25bdd67a6aceff05529a533260b661b07c2928
Reviewed-on: https://chromium-review.googlesource.com/c/1262740
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>