Defglstruct now makes full varjo structs along with its own foreign
struct allows for seamless integration between foreign-c and
foreign-glsl code. See below for example:

;; DEFINE THE STRUCT

CGL> (defglstruct vert
       (pos :vec3)
       (col :vec4))
VERT

;; USE IT IN A VARJO SHADER

CGL> (varjo:defshader :test ((a vert) &uniform (b vert))
       a)

;; SHADER CODE IS GENERATED

#version 330

struct vert {
    VEC3 POS;
    VEC4 COL;
};

layout(location=0) in vec3 _f_A_POS;
layout(location=1) in vec4 _f_A_COL;

uniform vert B;

void main() {
    A;
}