Implement Gaussian anti-aliasing

include/rng.h

@@ -10,8 +10,7 @@ typedef struct {
 } rng_t;
 
 rng_t rng_init(unsigned seed);
-
-double rng_double(rng_t *rng);
 vec3_t rng_vec3(rng_t *rng);
+vec3_t rng_gaussian_xy(rng_t *rng, double stddev);
 
 #endif