Fix outline size when using default style for ASS subtitles.
RTS: Add some rounding to ensure higher precision.
RTS: Remove work-around which make things only worse.

Fixes #4210
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.
Add floating point comparison function.