Disable cross-fading between tracks with different sample rates

Cross-fading between tracks with differing sample rates (eg 44.1kHz ripped from CD and 48kHz high quality downloads) appears to cause an alarming burst of white noise at full volume during the cross-fade period. This is the case for my Squeezebox Touch when using the analogue outputs, at least.

This modification is an extension of the method used for 'smart' cross-fade and temporarily disables cross-fading when transitioning between tracks with different sample rates.

I believe this might fix bug#1884 (http://bugs.slimdevices.com/show_bug.cgi?id=1884) -- a golden oldie that is more than seven years old.