Fix the renderList sort function to be truly stable

Multiple objects can have the same id. Using that id to compare the render
objects makes the sort function unstable. Use the webgl object's index in the
render list instead. That number is guaranteed to be unique.

This fixes #2966.

src/renderers/WebGLRenderer.js

@@ -4037,6 +4037,7 @@ THREE.WebGLRenderer = function ( parameters ) {
 			webglObject = renderList[ i ];
 			object = webglObject.object;
 
+			webglObject.id = i;
 			webglObject.render = false;
 
 			if ( object.visible ) {
@@ -4064,8 +4065,6 @@ THREE.WebGLRenderer = function ( parameters ) {
 
 						}
 
-						webglObject.id = object.id;
-
 					}
 
 				}