Subject: modifying input parameters

(This message is being sent to all CPSC 102 Sec 1 students )

The base and direction vectors passed to all of the hits functions MUST
be treated as "read only" input parameter.  The find_closest_object()
function will pass these vector to ALL of the hits functions.

If one of the hits functions modifies them the whole system falls
apart :-(  as this student found out.

   //Compute location of hit point
     vec_scale(dist, dir, dir);
     vec_sum(base, dir, hp);