title: Enabling an extension on a Three.js shader
body: <p>How can I enable an extension on a Three.js shader?</p>
  
  <p>My code so far:</p>
  
  <p>getting extension:</p>
  
  <pre><code>var domElement = document.createElement( 'canvas' );
  var gl = domElement.getContext('webgl') || domElement.getContext('experimental-webgl');
  gl.getExtension('OES_standard_derivatives');
  </code></pre>
  
  <p>on my shader: </p>
  
  <pre><code>fragmentShader: [
      "#extension GL_OES_standard_derivatives : enable",
      "code..."
  ]...
  </code></pre>
  
  <p>The console output:<br/>
  WARNING: 0:26: extension 'GL_OES_standard_derivatives' is not supported<br/>
  ERROR: 0:32: 'dFdx' : no matching overloaded function found <br/>
  ERROR: 0:32: '=' :  cannot convert from 'const mediump float' to '2-component vector of float'<br/>
  ERROR: 0:33: 'dFdy' : no matching overloaded function found<br/>
  ERROR: 0:33: '=' :  cannot convert from 'const mediump float' to '2-component vector of float'<br/></p>
  
  <p>After reading <a href="https://github.com/mrdoob/three.js/issues/1621" rel="noreferrer">this issue on github</a>, I tried this example: From <a href="http://jsfiddle.net/VJca4/" rel="noreferrer">http://jsfiddle.net/VJca4/</a> I get these errors</p>
  
  <p>WARNING: 0:27: extension 'GL_OES_standard_derivatives' is not supported<br/>
  ERROR: 0:30: '=' :  cannot convert from 'const mediump float' to '2-component vector of float'<br/>
  ERROR: 0:31: 'dFdx' : no matching overloaded function found<br/>
  ERROR: 0:31: '=' :  cannot convert from 'const mediump float' to '2-component vector of float'<br/></p>