rtices all have Z values.
 The containing triangle must not be degenerate
 (in other words, the three vertices must enclose a 
 non-zero area).
<P>
<DD><DL>
<DT><B>Parameters:</B><DD><CODE>p</CODE> - the point to interpolate the Z value of<DD><CODE>v0</CODE> - a vertex of a triangle containing the p<DD><CODE>v1</CODE> - a vertex of a triangle containing the p<DD><CODE>v2</CODE> - a vertex of a triangle containing the p
<DT><B>Returns:</B><DD>the interpolated Z-value (height) of the point</DL>
</DD>
</