Use more numerically robust algorithm to compute QBezier::pointAt().

Use more numerically robust algorithm to compute QBezier::pointAt().

QBezier::pointAt() could potentially return values outside the bezier's
bounds, even when the bezier was a straight horizontal line. For
example, with y = 0.5, it would produce y=0.5 or y=0.49999999999999 for
different values of t, which when rounded cause jittering in a QML
PathView.

Task-number: QTBUG-17007
Task-number: QTBUG-18133
Cherry-pick-of: 8b66982ec7b4b5d2071931c288973dce73dc9875
Change-Id: I4ecac7b9085aaaaaaaaaaaaaaaaaaaaaa7d7b0bc
Reviewed-on: http://codereview.qt.nokia.com/2467
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>