Add support for alphaTest parameter in JSON materials

src/loaders/MaterialLoader.js

@@ -50,6 +50,7 @@ THREE.MaterialLoader.prototype = {
 		if ( json.opacity !== undefined ) material.opacity = json.opacity;
 		if ( json.transparent !== undefined ) material.transparent = json.transparent;
 		if ( json.wireframe !== undefined ) material.wireframe = json.wireframe;
+		if ( json.alphaTest !== undefined ) material.alphaTest = json.alphaTest;
 
 		// for PointCloudMaterial
 		if ( json.size !== undefined ) material.size = json.size;