The 'busy' field in GpImage is used as an atomic variable.  The C11
standard (§5.1.2.4, paragraph 25) states that two conflicting actions to
a memory location shall be both atomic operations, or otherwise properly
synchronized; otherwise, it constitutes a data race.

However, select_frame_wic() performs a non-atomic access to the 'busy'
field on a GpImage that is potentially accessible by other threads.
This happens when select_frame_wic() copies new_image to the old image
object.  Although it does attempt to preserve the value of the 'busy'
field by setting new_image->busy = image->busy first, thereby
effectively assigning an identical value to the field, it is unclear
that this does not actually constitute a theoretical, if not practical,
data race.  This also prevents replacing the busy flag with a mutex or
other synchronization primitives.

Therefore, skip the 'busy' field when copying fields from the new image
to the original image object.