Make pixbuf-lowmem test work without vtable
Make pixbuf-lowmem test work without vtable

Make the custom malloc setup in this test work without the
no-longer-working g_mem_set_vtable, by overriding the weak
symbols for malloc, realloc, free directly. This will probably
only work with glibc, since we rely on the __libc- prefixed
versions of these functions.

Distrotech/gdk-pixbuf,Distrotech/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,Distrotech/gdk-pixbuf,GNOME/gdk-pixbuf,Distrotech/gdk-pixbuf,Distrotech/gdk-pixbuf