Subject: Use of the last_hit parameter is find_closest_object

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  obj_t *find_closest_object(
  list_t *lst,           /* Visible Object list          */
  double *base,          /* Base of ray                  */
  double *dir,           /* direction of ray             */
  obj_t  *last_hit,      /* -> object last hit (or NULL) */
  double *closest_dist); /* return dist to hitloc here   */

For diffuse illumination to work it is necessary to use
set the last_hit parameter to the "hitobj" when calling
find_closest_object().

Inside find_closest_object() you MUST NOT test the last_hit
object (because if you do you will find that the ray fired
from the hitobj toward the light hits the hitobj at a distance
0.0 and you will mistakenly conclude the hitobj is occluded)!