Make effectiveDevicePixelRatio() return a qreal.

Fractional scale factors are not as broken as previously
believed, especially for Qt Quick. Keep the door open
for finding a way to support it at some point in the
future.

Change-Id: Ifeadcc53175ac6c25ea0288d5fe1966e3de408f9
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>