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.