[PATCH] gnu: mesa: Move out of xorg.scm and into gl.scm

[John Darrington <jmd@gnu.org>]

The driver dependencies on mesa are probably correct.  

The history is interesting:  X11 of course predates OpenGL.  X11 provides a 2D
graphics system.  OpenGL provides a 3D system.  Early OpenGL implementations for 
*nix like systems used the X11 primitives for their implementation.  This was
found to be slow. Later versions bypassed X11 and rendered directly at the hardware
level.  Then, a number of X driver developers realised that much of the work 
they were doing had already been done by OpenGL implementations (such as Mesa).
So rather than doing it themselves, they decided simply to use Mesa.

In other words, whereas Mesa used to be a layer on top of X11, the situation has
reversed itself.  Today, X11 is an abstraction on top of Mesa.