title: using custom attribute in indexed BufferGeometry [three.js]
body: <p>I have an indexed bufferGeometry which have <code>position</code> , <code>appear</code> attributes
  I had set appear attribute to is the vertex appear in camera or not.
  I want to use it for check if my line should be render or not
  I want my line rendered if at least one of it's Vertices appear in camera.</p>
  
  <p>I had wrote this without indexing (using every vertex several times) before using  discard in fragment shader but now with indexing I have problem and they will not disappear
  Thanks in advance</p>