title: OpenGL 2d (orthographic) rendering for GUI, the modern way
body: <p>I want to render GUI components in my OpenGL program. They are still just simple textured (vbo) rectangles.
  I would like to get the following things done, the right way.</p>
  
  <ol>
  <li>Drawing using screen coordinates, or at least using a coordinate system that's not based on perspective-like floating points. For example: now the coordinate system is from -1f to 1f (left to right of the screen). It would be more logical to use screen/pixel coordinates.</li>
  <li>If it's easy to do, I'd like that the GUI doesn't stretch when the window (viewport) resizes.</li>
  </ol>
  
  <p>I know, previously you could do a lot using the deprecated function <code>glOrtho</code>. But since I want to do it the modern way, which is hopefully also better for performance, I don't know how to start.
  After searching on the internet, I came to the conclusion that I have to use a shader. I'm not very familiar with shaders.
  And another question: does performance increase when doing this using a shader?</p>