core: Always initialise default pixbuf loaders
The "run once" initialisation of pixbuf modules shipped with gdk-pixbuf itself would be skipped if an application was successfully calling gdk_pixbuf_init_modules() first, as this would set the internal list of file_formats to be non-NULL, and skip any initialisation of those modules.
This fix makes sure that pixbuf modules shipped with gdk-pixbuf are always initialised, regardless of whether gdk_pixbuf_init_modules() successfully initialised an application provided one.
Fixes: fd1376b7