virgl: support write-back with staged transfers

virgl: support write-back with staged transfers

We currently don't support writing to resources that uses a temporary
staging-resource to resolve the pixels. If a write-bit was set, we
forgot to perform a blit back to the old resource, followed by trying to
update the wrong resource, which lacks backing-storage. The end-result
would be that nothing useful happened.

This approach also fixes a few smaller bugs, like using the wrong box
(without x y and z zeroed out), which means a partial update of a
multisampled texture could result in the wrong part of the texture being
updated.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>