Don't return a copy of drawcalls in offsets getter

src/core/BufferGeometry.js

@@ -52,7 +52,7 @@ THREE.BufferGeometry.prototype = {
 	offsets: function () {
 
 		console.warn( 'THREE.BufferGeometry: .offsets has been renamed to .drawcalls.' );
-		return this.drawcalls.slice();
+		return this.drawcalls;
 
 	},