Rename rng_disc() to rng_plusminus()

include/rng.h

@@ -12,7 +12,7 @@ typedef struct {
 rng_t rng_init(unsigned seed);
 uint32_t rng_uint32(rng_t *rng);
 double rng_canon(rng_t *rng);
-double rng_disc(rng_t *rng);
+double rng_plusminus(rng_t *rng);
 vec3_t rng_vec3(rng_t *rng);
 vec3_t rng_gaussian_xy(rng_t *rng, double stddev);