Add support for LD-G

[DreckSoft]

Support for LD-G would be nice to process subtitles on mostly Japanese LDs.

LD-G is stored within the digital audio track like with CD-G. It can contain multiple subtitles. I don't know it the format is exactly the same as CD-G but should be similar.

For CD-G I found two projects dealing with it:
https://github.com/isaacbrodsky/cdgdeck
https://github.com/NotTheRealJoe/cdgtools

Some technical information:
https://jbum.com/cdg_revealed.html