add stream_decode_offset to DecoderPlugins

For sample accurate seeking, decoders need to know from which sample offset it needs to start outputting to the output thread.
Add offset to flushevent and hookup to input and decoder thread
[vorbis] implement new stream_decode_offset
Reset stream_decode_offset in init()
introduce new seeking plugin api
[cleanup] Remove unused variable
Implement stream_decode_offset in AAC decoder
Add debugging logging to stream decode offset handling in aac
add accurate seeking to opus
Merge branch 'feature-sample-accurate-seeking' into develop
[mad] don't read past end of decoder buffer
[mad] make some functions inline
[mp4] fix uninitialized variable use