Implement a VobSub decoder
Around https://docs.rs/vobsub/0.2.3/vobsub/, probably best as GstVideoDecoder subclass once we have it... or GstElement.
Needs some changes to expose the &[u8] based API that is currently internally, and accepts elementary stream packets.
The repository also has some SRT code, might be worth adding something around that too.
CC @emk who brought this to my attention on reddit