WIP: fixes for OpenGL 2.1

I've come to the conclusion that I should make some effort to get this running on my laptop (or get a better laptop :smile:). The "Images" commit is probably something you want anyway, the second I'm far less sure about. I basically know nothing about OpenGL, though I'm hoping to change that.

This has gotten me past a couple of errors. Where I'm stymied now is this error:
```C
#version 120

uniform vec4 bg_color;
uniform vec4 grid_color;
uniform vec3 grid_thickness;
uniform vec3 gridsteps;

varying vec3 vposition;

varying vec4 fragment_color;
#define M_PI 3.1415926535897932384626433832795
void main()
{
        vec3  v                 = vec3(vposition.xyz) * gridsteps * M_PI;
    vec3  f             = abs(cos(v));
    vec3  df            = fwidth(v);
    vec3  g             = smoothstep(vec3(1.0) - vec3(0.01), vec3(1.0), f) * df;
    float c             = max(g.x, max(g.y, g.z))*10;
    fragment_color      = mix(bg_color, vec4(vposition, 0.2), c);
}
ERROR: /home/tim/.julia/v0.4/GLPlot/src/shader/grid.frag
0:19(5): error: assignment to read-only variable 'fragment_color'
```
It seems like the [fix is to use `gl_FragColor`](http://stackoverflow.com/questions/24737705/opengl-shader-builder-errors-on-compiling). Am I on the right track? Is this likely to eventually result in success, and if so are you interested in supporting this? Or, is this just the first step along a great uglification of your beautiful, modern OpenGL packages? If the latter, I should just get a newer laptop.