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