Create a new gdk.Pixbuf containing a copy of src scaled to dest_width x dest_height. Leaves src unaffected. interp_type should be GDK_INTERP_NEAREST if you want maximum speed (but when scaling down GDK_INTERP_NEAREST is usually unusably ugly). The default interp_type should be GDK_INTERP_BILINEAR which offers reasonable quality and speed.
You can scale a sub-portion of src by creating a sub-pixbuf pointing into src; see Pixbuf.newSubpixbuf.
If dest_width and dest_height are equal to the src width and height, a copy of src is returned, avoiding any scaling.