UI|libappfw: Only allow UI downscaling on HiDPI displays

On a non-retina display, text would simply become unreadable. Better to use
the -dpi and -fontsize switches instead.

IssueID #2009

doomsday/sdk/libappfw/src/vrwindowtransform.cpp

@@ -66,6 +66,8 @@ DENG2_PIMPL(VRWindowTransform)
 
     float displayModeDependentUIScalingFactor() const
     {
+        if(GuiWidget::toDevicePixels(1) == 1) return 1.0f; // Not enough pixels for good-quality scaling.
+
         // Since the UI style doesn't yet support scaling at runtime based on
         // display resolution (or any other factor).
         return 1.f / Rangef(.5f, 1.0f).clamp((self.window().width() - GuiWidget::toDevicePixels(256.f)) /