Ensure wxBitmap::SetScaleFactor() doesn't change bitmap copies

Call AllocExclusive() to ensure that we're only changing the scale
factor of this object, and not of all the other objects using the same
data.

src/msw/gdiimage.cpp

@@ -346,6 +346,8 @@ void wxGDIImage::SetScaleFactor(double scale)
 {
     wxCHECK_RET( IsOk(), wxT("invalid bitmap") );
 
+    AllocExclusive();
+
     GetGDIImageData()->m_scaleFactor = scale;
 }