Thanks for the patch.
I'll add it to ImagingComponents unit commented for now for people who use latest Lazarus SVN versions and once new Lazarus release is out I'll comment the current code and make the new one a default (like I did with now current Lazarus release and RawImage interface changes).

Trying to compile imaging from svn with Lazarus from svn (rev 14368) under gtk gives an error due to internal changes in gtk widgetset.

Attached patch fixes it.