title: Threejs error when trying to implement ShaderToy
body: <p>first time poster... 
  I've been through SO looking at threejs/shadertoy stuff but haven't been able to find this particular issue.  I'm hoping someone can help.
  <br>
  <br>
  I'm attempting to bring this shadertoy:<a href="https://www.shadertoy.com/view/ltfXWr#" rel="nofollow noreferrer" title="terrain">https://www.shadertoy.com/view/ltfXWr#</a> into a simple html file and use threejs to display it.  I followed the general idea behind this: <a href="https://stackoverflow.com/questions/24820004/how-to-implement-a-shadertoy-shader-in-three-js">How to implement a ShaderToy shader in three.js?</a>
  <br>
  and added some uniforms at the top of my fragment shader to make up for the unique stuff (iResolution, iMouse,etc) Shadertoy uses. Outside of this I haven't adjusted any of the code from what you can see of the shadertoy source. <br>
  <br>
  When I run the code I I receive the following error: <br></p>
  
  <pre><code>THREE.WebGLProgram: shader error:  0 gl.VALIDATE_STATUS false gl.getProgramInfoLog invalid shaders  ERROR: 0:168: 'GL_OES_standard_derivatives' : extension is disabled&lt;br&gt;
  ERROR: 0:188: 'xy' :  field selection requires structure, vector, or matrix on left hand side &lt;br&gt;
  ERROR: 0:188: 'y' :  field selection requires structure, vector, or matrix on left hand side &lt;br&gt;
  ERROR: 0:188: 'res' : redefinition &lt;br&gt;
  ERROR: 0:189: 'y' :  field selection requires structure, vector, or matrix on left hand side &lt;br&gt;
  ERROR: 0:189: 'uv' : redefinition &lt;br&gt;
  ERROR: 0:191: 'xy' :  field selection requires structure, vector, or matrix on left hand side &lt;br&gt;
  ERROR: 0:191: 'xy' :  field selection requires structure, vector, or matrix on left hand side &lt;br&gt;
  ERROR: 0:191: 'constructor' : not enough data provided for construction
  </code></pre>
  
  <p><br>
  I'm just not entirely sure what to do at this point.  I'm new to a lot of this and it could well be that I'm missing something obvious.<br><br>
  My (not functioning, but you can see the error) codepen is here: <a href="http://codepen.io/ikimono/pen/RWVJYv" rel="nofollow noreferrer">http://codepen.io/ikimono/pen/RWVJYv</a></p>
  
  <p><br><br>Thank you in advance!</p>