+ * wpos = attr[FRAG_ATTRIB_WPOS] and MUST BE THE FIRST values in the
+ * vertex because of the tnl clipping code.
+
+ * wpos[0] and [1] are the screen-coords of SWvertex.
+ * wpos[2] is the z-buffer coord (if 16-bit Z buffer, in range [0,65535]).
+ * wpos[3] is 1/w where w is the clip-space W coord. This is the value
+ * that clip{XYZ} were multiplied by to get ndc{XYZ}.
+ *