Make all uses of MusicChunk safe.

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.

Replaces decoder_data() and others.

Replaces decoder_initialized().

Prepare for a Decoder API redesign based on an abstract class with
virtual methods.

This commit adds the basic infrastructure for reporting bugs from
DecoderAPI.cxx via DecoderThread.cxx to DecoderControl.

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

Must hold the Mutex to signal the Cond object safely.