copy material index of groups in BufferGeometry copy()

src/core/BufferGeometry.js

@@ -1003,7 +1003,7 @@ THREE.BufferGeometry.prototype = {
 		for ( var i = 0, l = groups.length; i < l; i ++ ) {
 
 			var group = groups[ i ];
-			this.addGroup( group.start, group.count );
+			this.addGroup( group.start, group.count, group.materialIndex );
 
 		}