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.

src/osx/core/bitmap.cpp

@@ -75,6 +75,8 @@ public:
     int GetBytesPerRow() const;
     bool HasAlpha() const;
     WXImage GetImage() const;
+
+    void SetScaleFactor(double scale) { m_scaleFactor = scale; }
     double GetScaleFactor() const { return m_scaleFactor; }
 
     const void *GetRawAccess() const;
@@ -1389,6 +1391,13 @@ int wxBitmap::GetWidth() const
    return GetBitmapData()->GetWidth() ;
 }
 
+void wxBitmap::SetScaleFactor(double scale)
+{
+    wxCHECK_RET( IsOk(), wxT("invalid bitmap") );
+
+    return GetBitmapData()->SetScaleFactor(scale) ;
+}
+
 double wxBitmap::GetScaleFactor() const
 {
     wxCHECK_MSG( IsOk(), -1, wxT("invalid bitmap") );