Playlist: reset song priority on playback

A priority should be a volatile thing: it should schedule a song for
playing back once, but it should not affect the next time the queue
gets played.

NEWS

@@ -7,6 +7,7 @@ ver 0.20 (not yet released)
   - ape: drop support for non-standard tag "album artist"
 * output
   - pulse: set channel map to WAVE-EX
+* reset song priority on playback
 
 ver 0.19.5 (not yet released)
 * decoder

doc/protocol.xml

@@ -240,7 +240,8 @@
         bigger than the priority of the current song.  Decreasing the
         priority of a song will moved it farther to the end of the
         sequence.  Changing the priority of the current song has no
-        effect on the sequence.
+        effect on the sequence.  During playback, a song's priority is
+        reset to zero.
       </para>
     </section>
   </chapter>

src/queue/Playlist.cxx

@@ -64,6 +64,10 @@ void
 playlist::SongStarted()
 {
 	assert(current >= 0);
+
+	/* reset a song's "priority" when playback starts */
+	if (queue.SetPriority(queue.OrderToPosition(current), 0, -1, false))
+		OnModified();
 }
 
 inline void