Accept a Bitmap as Reference in ImageData

The ImageData takes shared ownership of the Bitmap. Therefore,
passing a Reference by value is fine (see GotW 91 solution 3 (e),
http://herbsutter.com/2013/06/05/gotw-91-solution-smart-pointer-parameters/).
Accept a Bitmap as const ref in Image

Neither does the function take shared ownership nor does it change the
reference. A copy is created. Therefore, const ref is fine (see GotW 91
solution 3 (b),
http://herbsutter.com/2013/06/05/gotw-91-solution-smart-pointer-parameters/).