Add wxDisplay::GetScaleFactor()

This is conceptually the same as the ratio of the current DPI to the
standard one, but can be implemented more directly for wxGTK3 and wxOSX
(although the latter doesn't implement it yet).

include/wx/display.h

@@ -102,6 +102,9 @@ public:
         return wxSize(GetStdPPIValue(), GetStdPPIValue());
     }
 
+    // get the scaling used by this display
+    double GetScaleFactor() const;
+
     // name may be empty
     wxString GetName() const;