Imagination GL drivers are buggy with context switching.
We need to ubind fbo to workaround a crash in the driver.

Bug: chromium:1187513
Change-Id: Ie5142b5f43a3f23a9743ba7af0d90567fedd00d3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2841083Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Peng Huang <penghuang@chromium.org>