gluUnProject - map window coordinates	to object coordinates

	  GLint	gluUnProject( GLdouble winX,
			      GLdouble winY,
			      GLdouble winZ,
			      const GLdouble *model,
			      const GLdouble *proj,
			      const GLint *view,
			      GLdouble*	objX,
			      GLdouble*	objY,
			      GLdouble*	objZ )

	  winX,	winY, winZ
			  Specify the window coordinates to be mapped.

	  model		  Specifies the	modelview matrix (as from a
			  glGetDoublev call).

	  proj		  Specifies the	projection matrix (as from a
			  glGetDoublev call).

	  view		  Specifies the	viewport (as from a
			  glGetIntegerv	call).

	  objX,	objY, objZ
			  Returns the computed object coordinates.

	  gluUnProject maps the	specified window coordinates into
	  object coordinates using model, proj,	and view.  The result
	  is stored in objX, objY, and objZ. A return value of GL_TRUE
	  indicates success; a return value of GL_FALSE	indicates

	  To compute the coordinates (objX, objY, and objZ),
	  gluUnProject multiplies the normalized device	coordinates by
	  the inverse of model*proj as follows:

				     |			     |
				     |_________________	     |
		  (	  )	     |	   view[2]	- 1  |
		  | objX  |	     |			     |
		  | objY  | = INV(PM)|_________________	- 1  |
		  |	  |	     |	   view[3]	     |
		  | objZ  |	     |	   2(winZ) - 1	     |
		  (  W	  )	     |			     |
				     (		1	     )

	  INV()	denotes	matrix inversion. W is an unused variable,
	  included for consistent matrix notation.

	  glGet, gluProject