* In Quake 3, by default use Mesa as the OpenGL implementation. But

  allow the OpenGL implementation to be overriden through the
  OPENGL_DRIVER environment variable.  If it is not set, we use the
  implementation installed in the profile
  /nix/var/nix/profiles/opengl, allowing easy late binding by the
  user, e.g.,

    $ nix-env -p /nix/var/nix/profiles/opengl -i nvidia-sys-opengl

  might install the NVidia OpenGL implementation.

  The code that does this is not specific to Quake 3: it has been
  factored out into build-support/opengl/mesa-switch.sh.  Presumably
  any application that requires hardware-accelerated OpenGL needs it.

* Add the Quake 3 demo to the cache.

svn path=/nixpkgs/trunk/; revision=4612