title: Strange and annoying GLSL error
body: <p>My vertex shader looks as follows:</p>
  
  <pre><code>#version 120
  uniform float m_thresh;
  
  varying vec2 texCoord;
  
  void main(void)
  {   
      gl_Position = ftransform();
      texCoord = gl_TexCoord[0].xy;
  }
  </code></pre>
  
  <p>and my fragment shader:</p>
  
  <pre><code>#version 120
  uniform float m_thresh;
  uniform sampler2D grabTexture;
  
  varying vec2 texCoord;
  
  void main(void)
  {
      vec4    grab = vec4(texture2D(grabTexture, texCoord.xy));
      vec3    colour = vec3(grab.xyz * m_thresh);
      gl_FragColor = vec4( colour, 0.5 );
  }
  </code></pre>
  
  <p>basically i am getting the error message "Error in shader -842150451 - 0&lt;9> : error C7565: assignment to varying 'texCoord'"</p>
  
  <p>But I have another shader which does the exact same thing and I get no error when I compile that and it works!!!</p>
  
  <p>Any ideas what could be happening?</p>