ShaderMaterial: Fixed copy of defines.

src/materials/ShaderMaterial.js

@@ -93,7 +93,7 @@ ShaderMaterial.prototype.copy = function ( source ) {
 
 	this.uniforms = UniformsUtils.clone( source.uniforms );
 
-	this.defines = source.defines;
+	this.defines = Object.assign( {}, source.defines );
 
 	this.wireframe = source.wireframe;
 	this.wireframeLinewidth = source.wireframeLinewidth;