title: Drawing to a FrameBuffer causes Y to be inverted
body: <p>I want to use a framebuffer to create an image using a shader program.
  This works well, except that the Y is inverted.
  It seems that the bottom left corner becomes the top left corner.
  Why does this happen? Is this normal?
  Does this happen on all android devices?  </p>
  
  <p>Inside my shader program <code>gl_FragCoord</code> will have inverted Y, depending on if i draw to the framebuffer or to the screen directly.</p>