title: webgl readpixels is always returning 0,0,0,0
body: <p>I am trying to do picking in WebGl. I have two shapes rendered along with different texture mapped on each. I am trying to grab pixel on certain co-ordinates. Here is the example.</p>
  
  <pre><code>var pixelValues = new Uint8Array(4);
  gl.readPixels(10, 35, 1, 1, gl.RGBA, gl.UNSIGNED_BYTE, pixelValues);
  console.log(pixelValues);
  </code></pre>
  
  <p>But pixelValues always contain [0,0,0,0]. What am I doing wrong? Do I need to do something related to framebuffer?</p>