Returning the property before the window is mapped could lead to
confusing behavior, and in particular strange differences between
vo_vdpau and vo_opengl. (vo_opengl creates the window right at the
start, while vdpau waits until the first reconfigure event.) It might
even be possible that for vo_opengl random results were returned,
because the hidden window can have different placement than the actual,
final one on initial video reconfig.
Fix this by returning the property only if the window is considered
mapped. command.c handles this case specifically, and makes the property
unavailable, instead of returning an empty list.
2b0fce56a2