vo: reject vo_redraw_frame() if no frames have been drawn

vo: reject vo_redraw_frame() if no frames have been drawn

vo_xv crashed if existing frames had been lost due to a config() call
in the middle of a file and vo_redraw_frame() was called. Add checks
to reject vo_redraw_frame() unless at least one frame has been flipped
after the the last configuration change, so individual VOs do not have
to deal with this case.