MeshStandardMaterial: metalnessMap now modulates metalness

src/renderers/shaders/ShaderChunk/metalnessmap_fragment.glsl

@@ -3,6 +3,6 @@ float metalnessFactor = metalness;
 #ifdef USE_METALNESSMAP
 
 	vec4 texelMetalness = texture2D( metalnessMap, vUv );
-	metalnessFactor = texelMetalness.r; // note equality, not *=
+	metalnessFactor *= texelMetalness.r;
 
 #endif