Store 'is_odd_frame' and 'frame number' separately in rng_seed.

Make room for that extra bit by using few less bits for the x/y values
crammed in there as well.
(And replace magic shift values with constants while we're at it.)

src/refresh/vkpt/shader/path_tracer_rgen.h

@@ -39,6 +39,14 @@ uniform accelerationStructureEXT topLevelAS[TLAS_COUNT];
 
 #define DESATURATE_ENVIRONMENT_MAP 1
 
+/* RNG seeds contain 'X' and 'Y' values that are computed w/ a modulo BLUE_NOISE_RES,
+ * so the shift values can be chosen to fit BLUE_NOISE_RES - 1
+ * (see generate_rng_seed()) */
+#define RNG_SEED_SHIFT_X        0u
+#define RNG_SEED_SHIFT_Y        8u
+#define RNG_SEED_SHIFT_ISODD    16u
+#define RNG_SEED_SHIFT_FRAME    17u
+
 #define RNG_PRIMARY_OFF_X   0
 #define RNG_PRIMARY_OFF_Y   1
 #define RNG_PRIMARY_APERTURE_X   2
@@ -171,7 +179,8 @@ get_hit_barycentric(RayPayloadGeometry rp)
 float
 get_rng(uint idx)
 {
-	uvec3 p = uvec3(rng_seed, rng_seed >> 10, rng_seed >> 20);
+	uvec3 p = uvec3(rng_seed >> RNG_SEED_SHIFT_X, rng_seed >> RNG_SEED_SHIFT_Y, rng_seed >> RNG_SEED_SHIFT_ISODD);
+	p.z = (p.z >> 1) + (p.z & 1);
 	p.z = (p.z + idx);
 	p &= uvec3(BLUE_NOISE_RES - 1, BLUE_NOISE_RES - 1, NUM_BLUE_NOISE_TEX - 1);
 

src/refresh/vkpt/shader/primary_rays.rgen

@@ -101,9 +101,10 @@ void generate_rng_seed(ivec2 ipos, bool is_odd_checkerboard)
 	uint frame_offset = frame_num / NUM_BLUE_NOISE_TEX;
 
 	rng_seed = 0;
-	rng_seed |= (uint(ipos.x + frame_offset) % BLUE_NOISE_RES) <<  0u;
-	rng_seed |= (uint(ipos.y + (frame_offset << 4)) % BLUE_NOISE_RES) << 10u;
-	rng_seed |= uint(frame_num + uint(is_odd_checkerboard)) << 20;
+	rng_seed |= (uint(ipos.x + frame_offset) % BLUE_NOISE_RES) << RNG_SEED_SHIFT_X;
+	rng_seed |= (uint(ipos.y + (frame_offset << 4)) % BLUE_NOISE_RES) << RNG_SEED_SHIFT_Y;
+	rng_seed |= uint(is_odd_checkerboard) << RNG_SEED_SHIFT_ISODD;
+	rng_seed |= uint(frame_num) << RNG_SEED_SHIFT_FRAME;
 
 	imageStore(IMG_ASVGF_RNG_SEED_A, ipos, uvec4(rng_seed));
 }