RTS: Change work-around for m_dstScreenSize to preserve proper subtitle
positioning and preserve subtitle font size for default style 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
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.