The Second Rule of Program Optimization (for experts only!)
           Don't do it yet. -- Michael Jackson

Concept 1.4: Push & Pop - Controlling Transformations
====================================================

Idea
====
In the previous examples by issuing translation commands
we have been creating, loading and multiplying the  
modelview matrices.  OpenGL provides two matrix stacks.
The modelview matrix stack and the projection matrix 
stack.  Commands used for translation, rotation, scaling
and looking at the object affect the modelview matrix 
stack.  Commands for specifying viewing volumes affect
the projection matrix stack.  

A stack of matrices is useful for constructing 
hierarchical models, which complicated objects are
constructed from simpler ones.  All matrix operations in
OpenGL deal with the current matrix or the top matrix on
the stack.   OpenGL provides two functions to control
what is on top of the stack.  
glPushMatrix() and  glPopMatrix() allow us to isolate 
transformations.  glPushMatrix() copies the current matrix 
and aadds the copy to the top of the stack, and 
glPopMatrix() which discards the top matrix on the stack.
In effect, glPushMatrix() means, "remember where you are"
and glPopMatrix() means "go back to where you were."

Experiment
==========
Try different values for both the cube and sphere.
Notice how by the introduction of glPushMatrix() 
before each translation call, and glPopMatrix() 
after rendering the objects, results in independent
translations for the cube and the sphere.  This
should answer the questions posed in Concept1b and
Concept1c.