This patch add another ARIB caption decoder using libaribcaption
external library.

Unlike libaribb24, it supports 3 types of subtitle outputs:
* text: plain text
* ass: ASS formatted text
* bitmap: bitmap image

Default subtitle type is ass as same as libaribb24.
Advantages compared with libaribb24 on ASS subtitle are:
* Subtitle positioning.
* Multi-rect subtitle: some captions are displayed at different
  position at a time.
* More stability and reproducibility.

To compile with this feature:
* libaribcaption external library has to be pre-installed.
  https://github.com/xqq/libaribcaption
* configure with `--enable-libaribcaption` option.

`--enable-libaribb24` and `--enable-libaribcaption` options are
not exclusive. If both enabled, libaribcaption precedes as
order listed in `libavcodec/allcodecs.c`.
Signed-off-by: rcombs <rcombs@rcombs.me>