This allows contexts to flush and update state when they are no longer
going to be current.

BUG=angleproject:2464

Change-Id: Ie577475a94090631a0208542b32a12a239bdeb75
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1553824Reviewed-by: Geoff Lang <geofflang@chromium.org>