When removing superseding updates, the superseded update was not
released, causing a memory leak.  This change also makes
SubresourceUpdate non-copyable and correctly implements the move
assignment operator such that swap between different update types are
correct.  As a result, the destructor can now ASSERT that the image is
not leaked.

Bug: chromium:1146516
Bug: chromium:1163354
Change-Id: I7531c91d8559c23b2e09159118fe645d12fc601f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2613201Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>