Add wxBitmap::operator=(wxImage) to wxMSW to resolve amboguities

Code doing "bitmap = image" compiled in the other ports, but not in
wxMSW with WXWIN_COMPATIBILITY_3_0==1 as there was an ambiguity between
converting wxImage to wxBitmap, as intended, or converting wxImage to
wxCursor and then assigning it to wxBitmap using operator=(wxCursor)
overload.

Resolve this ambiguity by providing operator=() overload taking wxImage
directly.

This fixes widgets sample compilation under MSW after the changes of
455a45f5a8

See https://github.com/wxWidgets/wxWidgets/pull/946

include/wx/msw/bitmap.h

@@ -103,6 +103,13 @@ public:
         (void)CopyFromCursor(cursor, wxBitmapTransparency_Auto);
     }
 
+#if wxUSE_IMAGE
+    wxBitmap& operator=(const wxImage& image)
+    {
+        return *this = wxBitmap(image);
+    }
+#endif // wxUSE_IMAGE
+
     wxBitmap& operator=(const wxIcon& icon)
     {
         (void)CopyFromIcon(icon);