This fixes spurious replay gain logs when the player inserts silence
chunks, because those silence chunks had no replay gain attached,
resetting the ReplayGainFilter state, flipping it forth and back.

This attribute is not particularly important, but it was
uninitialized.

This fixes an assertion failure caused by resuming playback before the
decoder has finished startup.

Eliminate a dependency from MultipleOutputs on PlayerControl.

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.

Prepare full C++ exception support in the decoder thread.

Fixes assertion failure in MultipleOutputs::Open() after seeking to a
failing song during playback.

If the decoder was just started, it already seeks to the desired
position.

If this gets called while seeking, do the right thing.  This moves the
elapsed_time setter from method Run(), which is now redundant.

Allows removing a few special cases in the player thread.

It is futile to check for decoder errors before the decoder has
finished startup.  At this time, it's unlikely that the decoder has
already failed.

.. and fix its API documentation.