lavc: mark bitmap based subtitles codecs as such.
lavc: support subtitles character encoding conversion.

TODO: bump lavc micro
lavfi/subtitles: split options between ass and subtitles.
lavfi/subtitles: support charenc option.
lavfi: rename vf_ass.c to vf_subtitles.c
lavfi/subtitles: set a different description between ass and subtitles.