Fixes bug #584877
Before this commit calling "gst_x_overlay_set_xwindow_id" more
than one time, had no effect.
It mainly affects the glimagesink implementation.
But on win32 (and CE), some stuff has to be done to
release the old parent.
And add a switchxoverlay example where the user
can click on left/right part of the main window to
switch the xoverlay.
