[plt-scheme] sgl - GL bindings for Scheme

I have added the sgl collection to the plt CVS tree and exp tagged it.  
This version of sgl is a complete re-write of the previous version 
which was linked to from my own web page.  While this new version is 
not yet complete, it already offers several advantages over the old 
version.

The new version incorporates two levels of bindings.  The lower level 
is based on Robert Kooima's GLSCM bindings and provides direct access 
to the C-language GL API.  The upper level provides a more Scheme-like 
interface very similar to the old sgl's interface.  The main change 
between the two versions is that functions like glVertex now have 2 
varieties instead of many, gl-vertex and gl-vertex-v.

Also, the new version is tested to work on MS Windows, Linux, Mac OS_X 
and X-windows in OS_X.

I urge current users to consider updating to the new version as they 
are able to.  The old version will remain available, but will not be 
maintained.

-Scott