Eevee: Add support for hair random property.

Do note that it does not match cycles implementation.

Also we could precompute the hash per strand before rendering but that would
suggest it's not per engine specific.

If we make the random value internal to blender then it won't be a matter
because other renderers will have access to the same value.

source/blender/draw/engines/eevee/shaders/bsdf_common_lib.glsl

@@ -143,6 +143,20 @@ vec2 mip_ratio_interp(float mip) {
 	float low_mip = floor(mip);
 	return mix(mipRatio[int(low_mip)], mipRatio[int(low_mip + 1.0)], mip - low_mip);
 }
+
+/* ------- RNG ------- */
+
+float wang_hash_noise(uint s)
+{
+	s = (s ^ 61u) ^ (s >> 16u);
+	s *= 9u;
+	s = s ^ (s >> 4u);
+	s *= 0x27d4eb2du;
+	s = s ^ (s >> 15u);
+
+	return fract(float(s) / 4294967296.0);
+}
+
 /* ------- Fast Math ------- */
 
 /* [Drobot2014a] Low Level Optimizations for GCN */

source/blender/draw/engines/eevee/shaders/lit_surface_frag.glsl

@@ -26,6 +26,7 @@ in vec3 hairTangent; /* world space */
 in float hairThickTime;
 in float hairThickness;
 in float hairTime;
+flat in int hairStrandID;
 
 uniform int hairThicknessRes = 1;
 #endif

source/blender/draw/engines/eevee/shaders/lit_surface_vert.glsl

@@ -35,11 +35,13 @@ out vec3 hairTangent;
 out float hairThickTime;
 out float hairThickness;
 out float hairTime;
+flat out int hairStrandID;
 #endif
 
 void main()
 {
 #ifdef HAIR_SHADER
+	hairStrandID = hair_get_strand_id();
 	vec3 pos, binor;
 	hair_get_pos_tan_binor_time(
 	        (ProjectionMatrix[3][3] == 0.0),

source/blender/gpu/shaders/gpu_shader_material.glsl

@@ -2486,7 +2486,7 @@ void node_hair_info(out float is_strand, out float intercept, out float thicknes
 	intercept = hairTime;
 	thickness = hairThickness;
 	tangent = normalize(hairTangent);
-	random = 0.0;
+	random = wang_hash_noise(uint(hairStrandID)); /* TODO: could be precomputed per strand instead. */
 #else
 	is_strand = 0.0;
 	intercept = 0.0;