title: 'texture2D' : No matching overloaded function found OpenGL ES2?
body: <p>I was working on a project, and for that project I  had to walk through a book called "OpenGL ES 2 For Android: A quick start guide".
  So when I got to texturing, I got the error of:</p>
  
  <pre><code>'texture2D' : No matching overloaded function found
  </code></pre>
  
  <p>When I compile the shader.
  The shader code:</p>
  
  <pre><code>// Fragment shader
  precision mediump float;
  
  uniform sampler2D u_TextureUnit;
  varying vec4 v_TextureCoordinates;
  
  void main()
  {
      gl_FragColor = texture2D(u_TextureUnit, v_TextureCoordinates);
  }
  
  // Vertex shader
  uniform mat4 u_Matrix;
  attribute vec4 a_Position;
  attribute vec4 a_TextureCoordinates;
  
  varying vec4 v_TextureCoordinates;
  
  void main()
  {
      gl_Position = u_Matrix * a_Position;
      v_TextureCoordinates = a_TextureCoordinates;
  }
  </code></pre>
  
  <p>I tried the same shaders for my project and for exactly the same code as in the book but it still gives me the same error when I compile the shader, and the viewport on the android device is blank, just the clear color I set is shown.</p>