It happend that the previously shown frame was still on screen
while the decoder reused it already and the result was a mixed
picture on screen.
Protection is easy: just keep a reference to previously shown
frame and it cannot be reused by the decoder until a frame
duration has passed which should be sufficient to see the
current frame on screen.
Such referencing has already been implemented although it was
not used for deinterlacing. Therefore it had been disabled to
get an additional frame for decoding in coping with dropped
frames.
The change reenables referencing the previously shown frame.

While disabling for progressive frames was ment to improve DVD
playback, it turned out that many TV broadcasters set this flag
too, although their content is not progressive.
Adding an option allows the user now to enable this feature when
the watched streams make correct use of this flag.
Another option allows to disable bob deinterlacing when a scaled
OSD is on screen. As bobbing adds some noise to horizontal lines
the OSD quality might be improved by enabling this option.

When xine-ui switches to fullscreen mode or back again, it will
change the drawable by sending XINE_GUI_SEND_DRAWABLE_CHANGED.
This may lead to BadDrawable X Errors when the video out thread
is still using the old drawable for drawing.
The changes below block XINE_GUI_SEND_DRAWABLE_CHANGED from
changing the drawable while it is used for drawing.