Fix Metal

libraries/pbrlib/genglsl/lib/mx39_microfacet_sheen.glsl

@@ -1,6 +1,15 @@
 #include "mx39_microfacet.glsl"
 #include "mx_microfacet_sheen.glsl"
 
+float mx_inversesqrt(float x)
+{
+#ifdef __METAL_VERSION__
+    return ::rsqrt(x);
+#else
+    return inversesqrt(x);
+#endif
+}
+
 // The following functions are adapted from https://github.com/tizian/ltc-sheen.
 // "Practical Multiple-Scattering Sheen Using Linearly Transformed Cosines", Zeltner et al.
 
@@ -33,7 +42,7 @@ mat3 mx39_orthonormal_basis_ltc(vec3 V, vec3 N, float NdotV)
     float lenSqr = dot(X, X);
     if (lenSqr > 0.0)
     {
-        X *= inversesqrt(lenSqr);
+        X *= mx_inversesqrt(lenSqr);
         vec3 Y = cross(N, X);
         return mat3(X, Y, N);
     }
@@ -83,7 +92,7 @@ vec3 mx39_zeltner_sheen_importance_sample(vec2 Xi, vec3 V, vec3 N, float roughne
     vec3 w = vec3(wo.x/aInv - wo.z*bInv/aInv, wo.y / aInv, wo.z);
 
     float lenSqr = dot(w, w);
-    w *= inversesqrt(lenSqr);
+    w *= mx_inversesqrt(lenSqr);
 
     // D(w) = Do(wo) . ||M.wo||^3 / |M|
     //      = Do(wo / ||M.wo||) . ||M.wo||^4 / |M|