Add rng_xy()

include/rng.h

@@ -14,6 +14,7 @@ uint32_t rng_uint32(rng_t *rng);
 double rng_canon(rng_t *rng);
 double rng_plusminus(rng_t *rng);
 vec3_t rng_vec3(rng_t *rng);
+vec3_t rng_xy(rng_t *rng);
 vec3_t rng_gaussian_xy(rng_t *rng, double stddev);
 
 #endif