title: Using gl_FragDepth in WebGL
body: <p>I am writing a 3D application in webgl, and need to provide my own depth data that is contained within a texture, my current code does this:</p>
  
  <p>VS:</p>
  
  <pre><code>    varying vec2 vUv;
  
      void main() {
          vUv = uv;
      }
  </code></pre>
  
  <p>FS:</p>
  
  <pre><code>    uniform sampler2D depthTex;
  
      varying vec2 vUv;
  
      void main() {
          gl_FragDepth = texture2D(depthTex, vUv).r;
      }
  </code></pre>
  
  <p>however <code>gl_FragDepth</code> is disabled in opengl-es (and therefor webgl) is there anyway to somehow enable it, or any way to provide my own depth data that doesnt involve heavy manipulation of render targets?</p>