<p>
3DCanvas is the carrier of the EZ graphics library.
All graphical primitives are realized through 3DCanvases.
To each 3DCanvas, there is an associated data structure called 
<em>the graphics context</em> which contains the entire state 
attributes of the graphics library. 
An application can have as many 3DCanvases as it needs. However,
there is only one <em> active 3DCanvas </em> at any given
instant.  Implicitly, the active 3DCanvas is
set to the last created 3DCanvas.
