Bug 1947 - Clicky play when dynamically changing number of playing channels

Rather than 'cut' (i.e. drop, not cut as in 'cut preview') channels immediately, we now only do so
if their last gain was already 0.0.  Instead they micro-fade out.  Later when the channels come back,
their last gain will be 0.0, so they will micro-fade in.

A comment explains that the code would be cleaner if we just computed gains, and not whether to
drop channels.

src/WaveTrack.cpp

@@ -237,6 +237,10 @@ void WaveTrack::SetOffset(double o)
    mOffset = o;
 }
 
+int WaveTrack::GetChannelIgnoringPan() const {
+   return mChannel;
+}
+
 int WaveTrack::GetChannel() const 
 {
    if( mChannel != Track::MonoChannel )