RTS: Don't remove subtitles effect when default style is applied.

"Default Style" option is to apply default style for subtitles and not
actually completely break them.

Fixes #4171
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.
Don't set subtitle again when manipulating style.

Fixes #2244
Use dynamic_cast instead of evil C style casting.
ISR: Disable "Default Style" menu item when not applicable.