sd_ass, sd_lavc: use the input video's pixel aspect for scaling subtitles.

The previous code used the output video's pixel aspect for stretching
purposes, breaking rendering with e.g. -vf scale in the chain. Now
subtitles are stretched using the input video's pixel aspect only,
matching the intentions of the original subtitle author.
sd_lavc, sd_spu: make dvdsub stretching conditional on --stretch-dvd-subs.

We found that the stretching - although it usually improves the looks of
the fonts - is incorrect.

On DVD, subtitles can cover the full area of the picture, and they have
the same pixel aspect as the movie itself.

Too bad many commercially released DVDs use bitmap fonts made with the
wrong pixel aspect (i.e. assuming 1:1) - --stretch-dvd-subs will make
these more pretty then.