Use EXT_host_image_copy if supported

[doitsujin]

Currently, only Lavapipe supports this to a sufficient degree, hence the draft.

Unfortunately, we cannot really use the extension on Nvidia in any meaningful way because host-visible images are only exposed on HVV, so we just end up disabling it there and keep using the fallback path. ReadFromSubresource seems to be uncommon enough that the quirks of our current fallback implementation don't really seem to matter anyway.

Should definitely hold off landing this until we get RADV support.