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.
Merge remote-tracking branch
'hickinbottoms/feature/fix-transition-sample-rates2' into public/7.9
Disable cross-fading between tracks with different sample rates

Merging patch provided by hickinbottoms. Thanks!