Register file types that can be handled by ImageMagick.
This function is called at startup, after loading the init file.
It registers the ImageMagick types returned by `imagemagick-filter-types'.
Registered image types are added to `auto-mode-alist', so that
Emacs visits them in Image mode. They are also added to
`image-type-file-name-regexps', so that the `image-type' function
recognizes these files as having image type `imagemagick'.
If Emacs is compiled without ImageMagick support, this does nothing.