Support cover art for audio files.

- Don't hide logo in audio mode.
- Preserve logo aspect ratio
- Try to find cover art file in current directory
- Don't try to auto-fit in audio mode
- Allow to go fullscreen in audio mode
- Don't limit logo size

Fixes #1531
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.