Make wxWindow::GetContentScaleFactor() useful for non-OSX platforms.

Return the ration of the current DPI to the standard one to allow using this
function to scale pixel values for the current screen resolution.

include/wx/window.h

@@ -533,9 +533,8 @@ public:
 
     // returns the magnification of the content of this window
     // eg 2.0 for a window on a retina screen
-    virtual double GetContentScaleFactor() const
-    { return 1.0; }
-    
+    virtual double GetContentScaleFactor() 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;

interface/wx/window.h

@@ -1126,10 +1126,18 @@ public:
        Returns the magnification of the backing store of this window, eg 2.0
        for a window on a retina screen.
 
+       This method can be used to adjust hard coded pixel values to the values
+       appropriate for the current screen resolution. E.g. instead of using
+       32px icons, which would look tiny on the high resolution (also known as
+       HiDPI or retina) displays, you should use
+       @code
+       wxRound(32*GetContentScaleFactor())
+       @endcode instead.
+
        @since 2.9.5
     */
-    virtual double GetContentScaleFactor() const;
-    
+    double GetContentScaleFactor() const;
+
     /**
         Returns the size of the left/right and top/bottom borders of this window in x
         and y components of the result respectively.

src/common/wincmn.cpp

@@ -791,6 +791,21 @@ wxSize wxWindowBase::DoGetBestSize() const
     return best;
 }
 
+double wxWindowBase::GetContentScaleFactor() const
+{
+    // Currently we don't support per-monitor DPI, so it's useless to construct
+    // a DC associated with this window, just use the global value.
+    //
+    // We also use just the vertical component of the DPI because it's the one
+    // that counts most and, in practice, it's equal to the horizontal one
+    // anyhow.
+    //
+    // Finally, we consider 96 DPI to be the standard value, this is correct
+    // at least for MSW, but could conceivably need adjustment for the other
+    // platforms.
+    return wxScreenDC().GetPPI().y / 96.;
+}
+
 // helper of GetWindowBorderSize(): as many ports don't implement support for
 // wxSYS_BORDER/EDGE_X/Y metrics in their wxSystemSettings, use hard coded
 // fallbacks in this case