RTS: Preserve proper subtitle positioning and constant font size for default style override along different subtitle files.

Fixes #1574
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
Don't set subtitle again when manipulating style.

Fixes #2244
ISR: Disable "Default Style" menu item when not applicable.
CSubPicQueueNoThread: Respect invalidation time.
Do not clear surface cache when max texture size has not been changed.

We were removing valid surfaces from the cache for PGS/DVB subtitles.
Basically surface cache was broken.
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.