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/msw/bitmap.h

@@ -186,6 +186,7 @@ public:
     void ResetAlpha() { UseAlpha(false); }
 
     // support for scaled bitmaps
+    virtual void SetScaleFactor(double WXUNUSED(scale)) { }
     virtual double GetScaleFactor() const { return 1.0; }
     virtual double GetScaledWidth() const { return GetWidth(); }
     virtual double GetScaledHeight() const { return GetHeight(); }