We're not permitted to make accesses to a resource while it's owned by an
external instance or API. Add some asserts to verify this.

Exempt images on platforms that don't implement external memory barriers.

Bug: angleproject:3289
Change-Id: I1de929f6a412bfe5c1b798eaa1dc401bbceb5b7f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2195685Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Michael Spang <spang@chromium.org>