First order
===========

* SampleBuf: make version without channels?
  (for more optimal buffer usage, see Delay and AGC filters for example)
* MPEGDemux: check marker bits
* Speakers: level: is it a part of speakers configuration? 
           :level may be part of Chunk()
           :do we need compare levels at Speakers::operator == (and !=)?
* Filter: add method to check if filter can proceed
  (even if filter supports given input format for filter chain or other
  complex filter it is possible that other settings prevents filter from
  working properly)
  [Source::get_output() may return error but filter can still proceed
   see AudioDecoder class as an example]
  current settings (required for FilterChain filter to validate its state)
  sink

Other
=====
* WriteBS: review class
* DRC should work on source channels, but we cannot simply place 
  AGC before Mixer and it is a problem.
  Overall DRC should look like 
  out = gain + k * level (logarithmic scale)
  But now gain is done in Mixer (before DRC):
  out = k * (level + gain)
  So DRC level shows something dependent on gain 
