Fix support for screen fractional scaling with Wayland

Fix support for screen fractional scaling with Wayland

The display was wrong when a backing store is in use (which is the case
with Wayland). To fix this in GuiWorkArea::Private::resetScreen(), the
pixelRatio is now stored as a double instead of an int.

Concerning support for QT_SCALE_FACTOR, the existing code was wrong
because this value is already taken into account in devicePixelRatioF
for Qt > 5.6 (no fractional scaling support before that). The
situation is as follows:

                   Qt < 5.6    5.6 <= Qt < 6  Qt 6
devicePixelRatio   int(ratio)  int(ratio)     ratio
devicePixelRatioF  N/A         ratio          ratio

So it is only between Qt 5.6 and Qt 6 that devicePixelRatioF() has to
be used instead of devicePixelRatio().
QGuiApplication::devicePixelRatio() does not have a 'F' version, it
always returns the real thing.

Fixes ticket #13039.