title: Emulating the GLSL flat qualifier in GLSL 120
body: <p>I want to emulate flat shading in a GLSL shader. Usually I would do that by passing the normal with a <code>flat</code> qualifier to prevent interpolation in the fragment shader.
  However, <code>flat</code> is not available on GLSL 120. </p>
  
  <p>How can I emulate it without duplicating the vertex data? (This is <em>not</em> premature optimization, rather post-mortem.)</p>