Today, the image_unlock() helper function has a data race due to
non-atomic write to GpImage's 'busy' flag which is accessible by other
threads.  Also, it lacks a release fence, which means that other threads
can observe the unlocked (busy = 0) state too early when the current
thread unlocks the image; specifically, the write to the 'busy' field of
the GpImage can be reordered before the last read/write to any other
fields of the same GpImage.

Fix this by replacing the 'busy' field of GpImage with SRWLOCK.