MP3Stream does not properly abstract over its source stream

[Drake53]

The Position and Length properties and the Seek() method of MP3Stream use the underlying source stream instead of the output PCM bytes. I know the summaries mention this, but it's still confusing and inconsistent with the Read() method.

[Drake53]

Made an example implementation here: Drake53@814acf0
I'm not really familiar with the MP3 format so I don't know if it's possible to determine the length and frame offsets without reading the entire file first, so for now I throw NotSupportedException for Length and I build the frame offsets dict as they're being read. This is enough to support loop points, for which I only needed a working Seek() implementation that can seek backwards.