Use FastATan() for LTC line lights

No visual difference.

Assets/ScriptableRenderLoop/ShaderLibrary/AreaLighting.hlsl

 float LineFpo(float tLDDL, float lrcpD, float rcpD)
 {
     // Compute: ((l / d) / (d * d + l * l)) + (1.0 / (d * d)) * atan(l / d).
-    return tLDDL + (rcpD * rcpD) * atan(lrcpD);
+    return tLDDL + (rcpD * rcpD) * FastATan(lrcpD);
 }
 
 float LineFwt(float tLDDL, float l)