Screen to World coordinate mapping

[James D. Meiss]

	I am just learning how to program using gl on our new
personal Iris, and enjoying it for the most part!
	I am trying to read the position of the mouse to use
as input to my program. The problem is that GetValuator(MOUSEX)
returns the position in screen coordinates. How do I convert
this position to World coordinates?
	map2w seems to do this but only if I create an object.
It seems like there should be some simpler way. I use
ortho2(-1.,1.,-1.,1.) to set up my world coordinates.

	Thanks
	Jim Meiss
	jdm at boulder.colorado.edu