This leads to a problem where we're synching objects out of order. For
instance, when we call SetImage, we need to sync the pack state. But
SetImage can affect the FBO state, so we need to sync the FBO only
after we've finished with SetImage.

Fix this by using a mask of dirty objects to sync instead of all of
them, always. This also has the side effect of deferring some syncs
that don't have to be processed immediately.

BUG=angleproject:1260

Change-Id: I5678d8f967930d11b42a4309d209215be2bae963
Reviewed-on: https://chromium-review.googlesource.com/327259Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>