These changes store id3 chapter data in ID3v2ExtraMeta and introduce
ff_id3v2_parse_chapters to parse them into the format context if needed.

Encoders using ff_id3v2_read, which previously parsed chapters into the
format context automatically, were adjusted to call
ff_id3v2_parse_chapters.
Signed-off-by: wm4 <nfxjfg@googlemail.com>