This is done by resolving stencil into a temporary buffer and copying
that into the stencil aspect of the resolved image.

Bug: angleproject:3200
Change-Id: I29111b44db2cb093acc4544034fbe61178f055a1
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1635709
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>