Proposed WebGL example: multiple elements with text

Allow to specify renderOrder on Sprite instances.

This pull-request adds `renderOrder` functionality which  `Mesh` instances already have.

Let the CubeTextureLoader use the loading manager.

Actually use the loading manager in the CubeTextureLoader by passing it
to the internal ImageLoader.

WebGLState: Added clearDepth and clearStencil

Fix broken external link

Removal of THREE.MeshFaceMaterial in examples, docs and core

Refactor deprecated materials in some examples

fix statistics calculation for instanced geometry

Doc and examples

Didn't change the link to the source code at the end of
docs/api/extras/geometries/LatheGeometry.html.

Lathe with Vector2

Removed deprecated THREE.ImageUtils.loadTexture in all examples and docs

LatheGeometry modified to use Vector2 and allowing axe choice by the
called.
The axe is designed by a letter because I didn't know where I should put
a constance definition.
The loop that create the geometry is not optimized for speed => the axis
test is in the inner loop.
Should not break any existing code.
Doc and exemple not updated yet.

Support BufferGeometry, created by STLImporter

Removed deprecated THREE.ImageUtils.loadTextureCube in examples