Add wxWindow::GetDPIScaleFactor()

This function replaces some uses of GetContentScaleFactor(), where a
factor greater than 1 must be used even under the platforms not doing
any logical/physical pixel mapping, such as MSW.

For now GetContentScaleFactor() is still unchanged, but it will return 1
for such platforms in the future and adding GetDPIScaleFactor() allows
to avoid changing the behaviour of the code which relied on its current
behaviour.

include/wx/window.h

@@ -533,6 +533,10 @@ public:
     // e.g. 2.0 for a window on a retina screen
     virtual double GetContentScaleFactor() const;
 
+    // Return the ratio of the DPI used by this window to the standard DPI,
+    // e.g. 1 for standard DPI screens and 2 for "200% scaling".
+    double GetDPIScaleFactor() const;
+
     // return the size of the left/right and top/bottom borders in x and y
     // components of the result respectively
     virtual wxSize GetWindowBorderSize() const;
@@ -957,7 +961,7 @@ public:
         // DPI-independent pixels, or DIPs, are pixel values for the standard
         // 96 DPI display, they are scaled to take the current resolution into
         // account (i.e. multiplied by the same factor as returned by
-        // GetContentScaleFactor()) if necessary for the current platform.
+        // GetDPIScaleFactor()) if necessary for the current platform.
         //
         // To support monitor-specific resolutions, prefer using the non-static
         // member functions or use a valid (non-null) window pointer.