draw: add new debug code and comments in clip code template

In debug builds, set clipped vertex window coordinates to NaN values
to help debugging.  Otherwise, we're just leaving the coordinate in clip
space and it's invalid to use it later expecting it to be a window coord.

Reviewed-by: José Fonseca <jfonseca@vmware.com>

src/gallium/auxiliary/draw/draw_cliptest_tmp.h

@@ -129,6 +129,10 @@ static boolean TAG(do_cliptest)( struct pt_post_vs *pvs,
          need_pipeline |= out->clipmask;
       }
 
+      /*
+       * Transform the vertex position from clip coords to window coords,
+       * if the vertex is unclipped.
+       */
       if ((flags & DO_VIEWPORT) && mask == 0)
       {
 	 /* divide by w */
@@ -140,6 +144,17 @@ static boolean TAG(do_cliptest)( struct pt_post_vs *pvs,
 	 position[2] = position[2] * w * scale[2] + trans[2];
 	 position[3] = w;
       }
+#ifdef DEBUG
+      /* For debug builds, set the clipped vertex's window coordinate
+       * to NaN to help catch potential errors later.
+       */
+      else {
+         position[0] =
+         position[1] =
+         position[2] =
+         position[3] = 0.0f / 0.0f;
+      }
+#endif
 
       if ((flags & DO_EDGEFLAG) && ef) {
          const float *edgeflag = out->data[ef];