Well, you don't really need to mimic OO to make that code cleaner and less error prone.

Here's what he wrote in order to complain about it:

    if (mode == ambient) {
      // handle pixels as ambient...
      int steps = right.a.x - left.a.x;
      float dv = (right.a.ambientLight - left.a.ambientLight)/steps;
      float currentIntensity = left.a.ambientLight;
      for (i=left.a.x; i<right.a.x; i++) {
        WorkOnPixelAmbient(i, dv);
        currentIntensity+=dv;
      }
    } else if (mode == gouraud) {
      // handle pixels as gouraud...
      int steps = right.g.x - left.a.x;
      float dred = (right.g.red - left.g.red)/steps;
      float dgreen = (right.g.green - left.g.green)/steps;
      float dblue = (right.g.blue - left.g.blue)/steps;
      float currentRed = left.g.red;
      float currentGreen = left.g.green;
      float currentBlue = left.g.blue;
      for (j=left.g.x; i<right.g.x; j++) {
        WorkOnPixelGouraud(j, currentRed, currentBlue, currentGreen);
        currentRed+=dred;
        currentGreen+=dgreen;
        currentBlue+=dblue;
      }

Nobody should do that. We use a temporary variable of the correct type. For example this way (but it could also be done with pointers to access directly the union variable):

    switch(mode) {
      case AMBIENT: {
        PixelDataAmbient l = left.a;
        PixelDataAmbient r = right.a;
        int steps = r.x - l.x;
        float dv = (r.ambientLight - l.ambientLight)/steps;
        /* ... */
        break;
      }
      case GOURAUD: {
        PixelDataGouraud l = left.g;
        PixelDataGouraud r = right.g;
        int steps = r.x - l.x;
          /* ... */
          break;
        }
      /* ... */
      default:
        show_error("Illegal mode");
    }

There is no need to cart all the .a. and .g. around and clutter the code. And there is no possibility to accidentally access the wrong fields the wrong way: trying to access l.ambientLight in the second block (GOURAUD) will fail at compilation since a PixelDataGouraud doesn't have an ambientLight field. It is also not possible to do PixelDataGouraud l = left.a; since left.a is not a PixelDataGouraud.

Much cleaner, much more readable and avoids most of the possible errors he complains about.