Cleanup all the boilerplate, and port the function to use the auto
generated private helper. Remove the manual autocleanup declaration
since this is now done in the clutter-image.h header.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2355>
Mutter (and libmutter users) are the only users of this version of
cogl, and will more or less only use the cogl-1.0, cogl-2.0 and cogl
experimental API variants, and having the possibility of having
different API versions of the same API depending on what file includes
it is error prone and confusing. Lets just remove the possibility of
having different versions of the same API.
https://bugzilla.gnome.org/show_bug.cgi?id=768977
