Expose video metadata in the video addon

[SiegeLord]

I think we already read it in (into th_comment and vorbis_comment structures), just need a function to expose it. Maybe al_get_video_metadata(const VIDEO* video, char*** video_metadata, char*** audio_metadata), where both of the axes of the output parameters are zero-terminated.

Alternatively, could just make a al_get_video_theora_comment and al_get_video_vorbis_comment and return the original structures (this would be a platform style function, and would be placed in addons/video/include/allegro_ogv.h or something like that).