lavfi/subtitles: load attached fonts to libass.

Videos with complex typesetting usually have font files embedded
as attachment streams. vf_subtitles now finds all attachment
streams with a MIME type associated with fonts and loads them
to libass so it can use them for rendering.

The code was basically ported from mpv's loadfile.c@929793be7

Signed-off-by: Facundo Gaich <facugaich@gmail.com>