Update noise3Dgrad.glsl

Fixed an age-old bug that caused slight discontinuities along simplex boundaries.

src/noise3Dgrad.glsl

@@ -96,7 +96,7 @@ float snoise(vec3 v, out vec3 gradient)
   p3 *= norm.w;
 
 // Mix final noise value
-  vec4 m = max(0.6 - vec4(dot(x0,x0), dot(x1,x1), dot(x2,x2), dot(x3,x3)), 0.0);
+  vec4 m = max(0.5 - vec4(dot(x0,x0), dot(x1,x1), dot(x2,x2), dot(x3,x3)), 0.0);
   vec4 m2 = m * m;
   vec4 m4 = m2 * m2;
   vec4 pdotx = vec4(dot(p0,x0), dot(p1,x1), dot(p2,x2), dot(p3,x3));