This requires re-adding the reference to struct DecoderControl, which
was removed recently by commit
9f14e7a9

Make all uses of MusicChunk safe.

This tiny amount of overhead allows omitting the MusicBuffer in
Clear().

This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().

The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.

Instead of passing whole chunks to the MusicPipe and checking the
end_time after each chunk, truncate the last chunk if it would exceed
the end_time.  This requires keeping track of the absolute PCM frame
number.

This fixes a problem with gapless CUE song transitions: a small part
of the following song was always played twice.

Closes #113

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

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.