EEVEE-Next: Reduce volume probe blending and remove remapping

See 118342

source/blender/draw/engines/eevee_next/shaders/eevee_lightprobe_volume_eval_lib.glsl

@@ -150,9 +150,7 @@ SphericalHarmonicL1 lightprobe_irradiance_sample(
       index = i;
 #ifdef IRRADIANCE_GRID_SAMPLING
       float distance_to_border = reduce_min(min(lP, vec3(grids_infos_buf[i].grid_size) - lP));
-      if (distance_to_border < random) {
-        /* Remap random to the remaining interval. */
-        random = (random - distance_to_border) / (1.0 - distance_to_border);
+      if (distance_to_border < random * 0.5 /* Half cell blending. */) {
         /* Try to sample another grid to get smooth transitions at borders. */
         continue;
       }