title: ThreeJS, Multiple uv sets for 1 geometry
body: <p>I would like to build a vertex shader with 1 texture map but multiple uv sets.
  So far, I stored the differents UV sets in <code>FaceVertexUvs[0]</code> and <code>FaceVertexUvs[1]</code>.</p>
  
  <p>However, in the vertex shader, I can access only the first uv set using "vUv".</p>
  
  <pre><code>varying float vUv;
  void main() {
  (...)
  vUv = uv;
  (...)
  }
  </code></pre>
  
  <p>It seems like 'uv' is something magical from threejs. Does something like uv2, uv3 exists? I need something to access the uv mapping in <code>FaceVertexUvs[1]</code>.</p>
  
  <p>My goal is to build a house with wall using part of a texture, and windows in another part of the same texture, and blend both.</p>
  
  <p>Is it the correct way to do it? In which part of three.js source code is that magical 'uv' set?</p>