title: Flip upside down vertex shader (GLES)
body: <p>Given the next vertex shader, what is the simplest, most efficient and fastest way to flip the coordinates upside down, so the fragment shader will produce and upside down image?</p>
  
  <pre><code>attribute vec4 a_position;
  attribute vec2 a_texcoord;                                                  
  varying vec2 v_texcoord;
  
  void main()
  {
      v_texcoord = a_texcoord.st;
      gl_Position = a_position;
  }
  </code></pre>