Vinyl pitch fix 

this should fix bug https://bugs.launchpad.net/mixxx/+bug/1406131

@ywwg Now the keycontrol takes the rate value from VC in case of VC enabled.
This should fix the issue, but I cannot test it. 
Please have a look.
qFuzzyCompare does not work for values that are 0 (see documentation). 

It also hides what's really going on behind a Qt library function we don't control. Please use an explicit epsilon instead so that we document how close pitch ratio and speed need to be.