Context now holds it's own Caps, Extensions and TextureFormat caps so that it
can modify them based on client version or work-arounds.

BUG=angle:658

Change-Id: Id71b6c89b7aa36e1f3dc42b0e4720eaed1851fb9
Reviewed-on: https://chromium-review.googlesource.com/206480Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>