We only need to call the syncState for samplers when they are dirty.
Also includes changes to refactor out the sampler sync in GL. Adds
observer bindings so sampler sync is handled correctly in resource
sharing scenarios.

Bug: angleproject:2763
Change-Id: I762f0738ee7572ae29ce6bd5384a30aa9588c848
Reviewed-on: https://chromium-review.googlesource.com/1227797Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>