Other comments
Much optional code is protected by #ifdef statements:
- #if INTERP_Z - code for interpolating Z
- #if INTERP_RGG - code for interpolating RGB values
- #if INTERP_ALPHA - code for interpolating Alpha values
- #if INTERP_ST - code for interpolating texture coordinates
- #if INTERP_STW - code for interpolating texture coordinates with perspective correction
- #if INTERP_UV - code for interpolating UV values (only used for 3D texture maps).
Ignore code between #ifdefs if we are not using that feature! (ie. don’t care about texture coordinates for this exercise so ignore this code for now).