Allow canceling arbitrary active decoders

[sbooth]

This would probably require a timestamped ring buffer to work as expected. From @jlama

Let's say you have a big enough ring buffer that can hold 5s of audio:

1. Enqueue & play fileA
2. Enqueue fileB
3. fileA finishes decoding, fileB starts decoding. fileA will finish rendering in 5s
4. During that 5s window, queue order could be modified and fileB would need to be canceled (without affecting fileA). However fileB has already written some samples to the ring buffer...