ximagesink: generate reconfigure on window handle change

ximagesink: generate reconfigure on window handle change

When ximagesink is given a new window handle, it should check
its geometry and if the size of the new window differs from
the previous one, create reconfigure event in order to get
a chance to negotiate a more suitable image resolution with
the upstream elements.

We can't rely on receiving Expose or ConfigureNotify from
the X server for the newly assigned window, which would also
generate reconfigure.

https://bugzilla.gnome.org/show_bug.cgi?id=765424