Add dielectric material

include/rng.h

@@ -10,6 +10,7 @@ typedef struct {
 } rng_t;
 
 rng_t rng_init(unsigned seed);
+double rng_canon(rng_t *rng);
 vec3_t rng_vec3(rng_t *rng);
 vec3_t rng_gaussian_xy(rng_t *rng, double stddev);