title: correct glsl affine texture mapping
body: <p>i'm trying to code correct 2D affine texture mapping in GLSL.</p>
  
  <p>Explanation:</p>
  
  <p><img src="https://i.stack.imgur.com/8JS1D.jpg" alt=""></p>
  
  <p>...NONE of this images is correct for my purposes.  Right (labeled Correct) has perspective correction which i do not want.  So this: <a href="http://home.xyzw.us/~cass/qcoord/" rel="noreferrer">Getting to know the Q texture coordinate</a> solution (without further improvements) is not what I'm looking for.</p>
  
  <p>I'd like to simply "stretch" texture inside quadrilateral, something like this:</p>
  
  <p><img src="https://i.stack.imgur.com/V2KCQ.jpg" alt="enter image description here"></p>
  
  <p>but composed from two triangles.  Any advice (GLSL) please?</p>