Work around QPaintDevice::metric's int return type
by adding a new metric that returns a scaled devicePixelRatio.
Choose a scale factor that gives us more than enough
range.

The QPaintDevice::devicePixelRatio() convenience accessor
is public API and can unfortunately not be changed
to return a qreal. Add devicePixelRatioF() which
returns the (unscaled) devicePixelRatio.

Change all call sites of QPaintDevice::devicePixelRatio()
to use QPainDevice::devicePixelRatioF().

Task-number: QTBUG-46615
Change-Id: I97ec4000fe379b7ff5e1624a871ae2512790aad9
Reviewed-by: Paul Olav Tvete <paul.tvete@theqtcompany.com>
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>