Obsoletes the same variable from AudioConfig.cxx.

Don't use the global variables in class DecoderBridge; instead,
forward these values to the decoder thread via PlayerControl and
DecoderControl.

Replaces Reset() and eliminates the AudioFormat parameters from the
Convert() method.

Reduce header dependencies and allow it to be nullptr to disable it.

Remove the "do_wait" parameter which is always true.  Check only
command==NONE and merge the "return" statements.

Nothing of interest happens here.  No need to wake up the player.

Wake up the PlayerThread only if it is really waiting for the decoder.
This greatly reduces the number of system calls in the DecoderThread.

Must hold the Mutex to signal the Cond object safely.

Merge duplicate code.

Hide the definition from C code, to prepare the transition to C++.