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++.

This is not necessary since all relevant input plugins have been moved
to the I/O thread, and there is no remaining useful buffer()
implementation.  This also fixes a busy loop when playing radio.

Remove the decoder dependency on player_control.  All player_control
was needed for is to signal the player thread, and we can do that with
a simple GCond as well.

Allocate a player_control object where needed, and pass it around.
Each "client" object is associated with a "player_control" instance.

This prepares multi-player support.

Apply the replay gain in the output thread.  This means a new setting
will be active instantly, without going through the whole music pipe.
And we might have different replay gain settings for each audio output
device.

This fixes a regression: a boolean value was returned from
decoder_input_buffer(), but the caller chose to do a "<= 0"
comparison.