avcodec/png: remove obsolete comments

Signed-off-by: Paul B Mahol <onemda@gmail.com>
avformat/srtdec: skip initial random line breaks.

I found a bunch of (recent) SRT files in the wild with 3 to 10 line
breaks at the beginning.
avformat/subtitles: add a next line jumper and use it.

This fixes a bunch of possible overread in avformat with the idiom p +=
strcspn(p, "\n") + 1 (strcspn() can focus on the trailing '\0' if no
'\n' is found, so the +1 leads to an overread).

Note on lavf/matroskaenc: no extra subtitles.o Makefile dependency is
added because only the header is required for ff_subtitles_next_line().

Note on lavf/mpsubdec: code gets slightly complex to avoid an infinite
loop in the probing since there is no more forced increment.
avformat/subtitles: support standalone CR (MacOS).

Recent .srt files with CR only were found in the wild.
avcodec/assenc: fix potential overread.
avcodec/srtdec: fix potential overread.
avformat/avidec: fix signedness of pointer type in get_stream_idx()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>