Add after playback command to turn off the monitor.

Fixes #1287
SubPic queue: Clear queue only when it's needed.

Time can decrease even within the same video frame and
it make no sense to purge whole queue, if we still can reuse buffered
subpic.
MainFrame: adjust auto-zoom logic for DVB.

We can't determine whether event is triggered after channel change or on
resolution change. Always restore default auto-fit factor.