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/gtk/bitmap.h

@@ -17,6 +17,10 @@ typedef struct _GdkPixbuf GdkPixbuf;
 class WXDLLIMPEXP_FWD_CORE wxPixelDataBase;
 class WXDLLIMPEXP_FWD_CORE wxCursor;
 
+#ifdef __WXGTK3__
+    #define wxHAS_BITMAP_SCALE_FACTOR
+#endif
+
 //-----------------------------------------------------------------------------
 // wxMask
 //-----------------------------------------------------------------------------
@@ -85,8 +89,9 @@ public:
         { return Create(sz.GetWidth(), sz.GetHeight(), depth); }
     bool Create(int width, int height, const wxDC& WXUNUSED(dc))
         { return Create(width,height); }
-#ifdef __WXGTK3__
+#ifdef wxHAS_BITMAP_SCALE_FACTOR
     virtual bool CreateScaled(int w, int h, int depth, double scale) wxOVERRIDE;
+    virtual void SetScaleFactor(double scale) wxOVERRIDE;
     virtual double GetScaleFactor() const wxOVERRIDE;
 #endif