Unify sniff buffer size

GdkPixbufLoader was using 1k as a buffer size for sniffing file types,
and the gdk_pixbuf_new_from_file() family of functions was using 4k,
for no good reason. Unify this so we use the same buffer size
everywhere.
More sniff buffer size unification

Turns out gdk_pixbuf_animation_new_from_file() was also
using a different size.