player_thread: drain audio outputs at the end of the playlist

When there's no queued song, and the current one has finished playing,
first make sure that the hardware outputs have really finished playing
the last chunk: call the drain() method in all audio outputs.  Without
this patch, MPD stopped playback shortly before the ALSA sound card
had finished playing.

NEWS

@@ -43,6 +43,8 @@ ver 0.16 (20??/??/??)
   - httpd: bind port when output is enabled
   - wildcards allowed in audio_format configuration
   - consistently lock audio output objects
+* player:
+  - drain audio outputs at the end of the playlist
 * mixers:
   - removed support for legacy mixer configuration
   - reimplemented software volume as mixer+filter plugin

src/player_thread.c

@@ -874,8 +874,12 @@ static void do_play(struct decoder_control *dc)
 			/* check the size of the pipe again, because
 			   the decoder thread may have added something
 			   since we last checked */
-			if (music_pipe_size(player.pipe) == 0)
+			if (music_pipe_size(player.pipe) == 0) {
+				/* wait for the hardware to finish
+				   playback */
+				audio_output_all_drain();
 				break;
+			}
 		} else {
 			/* the decoder is too busy and hasn't provided
 			   new PCM data in time: send silence (if the