Compile following qreal -> float change in QtGui/math3d classes

This is needed following change

https://codereview.qt-project.org/#change,33148



Change-Id: I836d4037306d1efbff48668ff8dd2b7b5ce97516
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@nokia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>