Read QT_SCALE_FACTOR env. variable

Drop the "HIGHDPI" term
Don't scale the logical dpi

Scaling results in small fonts, which is undesirable.

From a theory point of view it can also be argued
that logical dpi does not change when scaling (physical
dpi does)
Test font pixel sizes as well.
Don't build an app bundle
Add logging via category "qt.scaling"

Change-Id: Ia94ba9a8961f1a31635135545ded0078a93de380
Create new backing store image on paintDevice()

Improve the layering separation between QtGui and
the platform plugin. QBackingStore no longer modifies
the image created by the platform plugin.
Use qHighDpitoDevicePixels
Add QPlatformWindow::window* accessors.

The platform plugins should use this to get geometry
and sizes in device pixels. (The corresponding QWindow
accessors return values in device-independent pixels).

This should be easier to use from the platform plugin
implementors point of view: there is no longer a need
to pick and use one of the qHighDpi* functions. We
can also hide implementation details like the "Constrained"
functions and normalization.
Cocoa: Use window* accessors
fixup cocoawin
XCB: Use window* accessors