MainFrm: Be more clever when to repaint video frame.

- Fix OSD rapaint in paused state with vanilla EVR.
- Some minor fixes in after playback handling.

Fixes #4807
DVD: Allow jumping in paused state.
Add an option to enable/disable cover-art support.

Fixes #4789
MainFrm: Don't search for cover-art if both author and current directory
is the same.

Ref: #4789