Sets a `GtkImage` to show a `GdkPixbuf`.
See [[email protected]_from_pixbuf] for details.
Note: This is a helper for [[email protected]_from_paintable], and you can't get back the exact pixbuf once this is called, only a paintable.
| this |
a `GtkImage` |
| pixbuf |
a `GdkPixbuf` or `NULL` |