Make snxtsct_area() expect correct range width and height

Make snxtsct_area() expect correct range width and height

snxtsct_area() computed width and height, overwriting the values
passed in, even though all but two callers passed correct values.  The
exceptions were snxtsct() in case NS_ALL, and snxtsct_all().  Change
them to pass correct values, and drop the recomputation from
snxtsct_area().  Simplifies the interface between snxtsct_area() and
its callers.