if a swapchain is shared between multiple resources, a deadlock and/or invalid
data will result from readback attempts if one resource holds a swapchain
image that the other resource must read from
to avoid this, allow accessing the acquiring resource directly since this is
the only sane way to perform readback
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27553>
Mike Blumenkrantz
d5d5f54fe9