Use a power of two division factor

Using a power of 2 instead of power of 10 means the result of the
division can be accurately represented as a floating point instead of
being an approximation that could lead to rounding errors.

Change-Id: I8910c06113ec6b69c60ff95d59894bfb56133186
Reviewed-by: Morten Johan Sørvig <morten.sorvig@theqtcompany.com>

src/gui/painting/qpaintdevice.h

@@ -81,7 +81,7 @@ public:
     int colorCount() const { return metric(PdmNumColors); }
     int depth() const { return metric(PdmDepth); }
 
-    static inline qreal devicePixelRatioFScale() {return 10000000.0; }
+    static inline qreal devicePixelRatioFScale() { return 0x10000; }
 protected:
     QPaintDevice() Q_DECL_NOEXCEPT;
     virtual int metric(PaintDeviceMetric metric) const;