StateManagerGL: Use dirty bits for framebuffer bindings. BUG=angleproject:2188 Change-Id: Icbe78f645b693a5b2ef555feffda0f618209e867 Reviewed-on: https://chromium-review.googlesource.com/757092 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Luc Ferron <lucferron@chromium.org>