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
2 jobs for wip/hadess/fix-external-pixbuf in 3 minutes and 52 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #236093
meson-fedora-x86_64

00:02:02

passed #236094
win32
msys2-mingw64

00:03:52