HDRenderPipeline: Add FastSign() function

Assets/ScriptableRenderPipeline/ShaderLibrary/Common.hlsl

     return pow(max(abs(base), float4(FLT_EPSILON, FLT_EPSILON, FLT_EPSILON, FLT_EPSILON)), power);
 }
 
+// Ref: https://twitter.com/SebAaltonen/status/878250919879639040
+// 2 mads (mad_sat and mad), faster than regular sign
+float3 FastSign(float x)
+{
+    return  saturate(x * FLT_MAX) * 2.0 - 1.0;
+}
+
 // ----------------------------------------------------------------------------
 // Texture utilities
 // ----------------------------------------------------------------------------

Assets/ScriptableRenderPipeline/ShaderLibrary/Packing.hlsl

     if (index == 2) quat = quat.xywz;
 
     float4 packedQuat;
-    packedQuat.xyz = quat.xyz * sign(quat.w) * sqrt(0.5) + 0.5;
+    packedQuat.xyz = quat.xyz * FastSign(quat.w) * sqrt(0.5) + 0.5;
     packedQuat.w = index / 3.0;
 
     return packedQuat;