Add wxBitmap::SetScaleFactor()

We need to be able to change the scale factor of the bitmaps returned by
wxBitmapBundle::GetBitmap(), so add a function allowing to do this.

Also add wxHAS_BITMAP_SCALE_FACTOR allowing to check whether this
function actually does something non-trivial and explain in the docs
that GetScaleFactor() always returns 1 on the platforms where this
symbol is not defined.

include/wx/osx/bitmap.h

@@ -13,6 +13,8 @@
 
 #include "wx/palette.h"
 
+#define wxHAS_BITMAP_SCALE_FACTOR
+
 // Bitmap
 class WXDLLIMPEXP_FWD_CORE wxBitmap;
 class wxBitmapRefData ;
@@ -239,6 +241,7 @@ public:
     void EndRawAccess();
 #endif
 
+    void SetScaleFactor(double scale) wxOVERRIDE;
     double GetScaleFactor() const wxOVERRIDE;
 
     void SetSelectedInto(wxDC *dc);