Grass first-pass

Following a thrust of vernal vigour, I thought I should change
everything to be set in summer. To this end, I added the basics of a
grass system using geometry shaders.
The change of direction was crap, but the code may be of use to add
winter grass instead.
The current system just spits out quads and samples a couple of textures
to determine where blades should be generated and their displacement.
The theory is that writing to the displacement texture should allow for
tracking of footsteps and wind.
The grass itself is comprised of nine sections that step around the
camera to hide the limits of the geometry shaders.