title: Glow effect on simple rectangle in openGL ES
body: <p>I would like to create a glow effect, on a rectangle:</p>
  
  <p>I don't really know where to start the fragment shader.</p>
  
  <p><img src="https://i.stack.imgur.com/K33wU.png" alt=".."></p>
  
  <p>Actually, I would like to achieve this effect on shapes (circles, polygons, rectangles). There is no real border color. There are just blury.</p>