gluPerspective - set up a perspective	projection matrix

	  void gluPerspective( GLdouble	fovy,
			       GLdouble	aspect,
			       GLdouble	zNear,
			       GLdouble	zFar )

	  fovy	  Specifies the	field of view angle, in	degrees, in
		  the y	direction.

	  aspect  Specifies the	aspect ratio that determines the field
		  of view in the x direction.  The aspect ratio	is the
		  ratio	of x (width) to	y (height).

	  zNear	  Specifies the	distance from the viewer to the	near
		  clipping plane (always positive).

	  zFar	  Specifies the	distance from the viewer to the	far
		  clipping plane (always positive).

	  gluPerspective specifies a viewing frustum into the world
	  coordinate system.  In general, the aspect ratio in
	  gluPerspective should	match the aspect ratio of the
	  associated viewport. For example, aspect=2.0 means the
	  viewer's angle of view is twice as wide in x as it is	in y.
	  If the viewport is twice as wide as it is tall, it displays
	  the image without distortion.

	  The matrix generated by gluPerspective is multipled by the
	  current matrix, just as if glMultMatrix were called with the
	  generated matrix.  To	load the perspective matrix onto the
	  current matrix stack instead,	precede	the call to
	  gluPerspective with a	call to	glLoadIdentity.

	  Given	f defined as follows:

			      f	= cotangent(____)
	  The generated	matrix is

		   (					    )
		   |  ______				    |
		   |  aspect  0	     0		  0	    |
		   |					    |
		   |	0     f	     0		  0	    |
		   |		 __________  ____________   |
		   |	0     0	 zNear-zFar   zNear-zFar    |
		   (					    )
			0     0	     -1		  0
	  Depth	buffer precision is affected by	the values specified
	  for zNear and	zFar.  The greater the ratio of	zFar to	zNear
	  is, the less effective the depth buffer will be at
	  distinguishing between surfaces that are near	each other.

				   r = _____

	  roughly log r	bits of	depth buffer precision are lost.
	  Because r approaches infinity	as zNear approaches 0, zNear
	  must never be	set to 0.

	  glFrustum, glLoadIdentity, glMultMatrix, gluOrtho2D