Documentation on how OpenGL calls work in Mojo applications

[James Robinson]

I've documented some of the details of how an OpenGL calls in a Mojo application ends up actually going somewhere here: https://docs.google.com/document/d/1ruH9_XE2RE3jkhdPODUZl6NDPbFvA8Aceu5nLfhLguY/edit.  I wrote this as part of describing a proposal for changing around how we deal with this but think it is useful on its own for folks trying to understand and debug the system as the combination of macro mangling and function pointer indirections can be really tricky to follow.  Please let me know if parts are unclear or inaccurate (heavens forbid!) or if there are parts of this system that you would find useful to describe in more depth.

- James