demux_libass: change how external ASS subtitles are loaded

Instead of parsing the ASS file in demux_libass.c and trying to pass the
ASS_Track to the subtitle renderer, just read all file data in
demux_libass.c, and let the subtitle renderer pass the file contents to
ass_process_codec_private(). (This happens to parse full files too.)

Makes the code simpler, though it also relies harder on the (messy)
probe logic in demux_libass.c.