Flickable fails to stop on a pixel boundary on OS X

I was checking the touch-related autotests on OS X due to the big parallel-event-propagation patch, and found the test for this in f67ba787 : tst_qquickflickable.cpp, flickOnRelease(). It fails on OS X; it stops flicking at 39.8xxx instead of 40.0. I put in an XFAIL for now, as part of this patch:

https://codereview.qt-project.org/#change,28377