GL backend: Only synchronize transform feedback state when it changes. BUG=angleproject:2188 Change-Id: I5bfcc038c887dde0770564d103eb3cb234b248c9 Reviewed-on: https://chromium-review.googlesource.com/794100 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>