Add wxDisplayImpl::GetScaleFactor() and implement it for wxGTK

We need to account for the scale factor under GTK+ (and, presumably,
under macOS) to compute the correct PPI value as it must use the number
of physical and not logical pixels.

include/wx/private/display.h

@@ -81,8 +81,11 @@ public:
     // return the depth or 0 if unknown
     virtual int GetDepth() const = 0;
 
-    // return the resolution of the display, uses GetSizeMM() by default but
-    // can be also overridden directly
+    // return the scale factor used to convert logical pixels to physical ones
+    virtual double GetScaleFactor() const { return 1.0; }
+
+    // return the resolution of the display, uses GetSize(), GetScaleFactor()
+    // and GetSizeMM() by default but can be also overridden directly
     virtual wxSize GetPPI() const;
 
     // return the physical size of the display or (0, 0) if unknown: this is