title: Is writing to `gl_Position` in Vertex Shader necessary when there is a Geometry Shader
body: <p>Suppose I have a geometry shader that computes its output <code>gl_Position</code> from some inputs other than <code>gl_in[].gl_Position</code>. If the previous pipelines stages (vertex and tessellation) do not write to their <code>out gl_Position</code>, does the action of the entire pipeline still remain well-defined?</p>
  
  <p>Or put it other way, does the value of <code>gl_Position</code> has any effect on the functioning of the GL before the completion of the Geometry shader? If not, it would mean I can simply use it as an extra slot for passing data without any special spatial interpretation between stages, right?</p>
  
  <p>(The question assumes OpenGL 4.5 forward profile.)</p>