title: OpenGL / GLSL line trails
body: <p>I'm trying to draw a line, and then have a trail that fades behind it as it moves.
  Think of the "Curves and Colors" screensaver from Windows 98. It's kind of a really simple-looking motion blur shader I guess?</p>
  
  <p>I've been searching for various things for hours with no luck. Maybe there's some term for it that I don't know or function I need to use?</p>
  
  <p>Can I do it with GLSL and save the previous points that the line was at, and then fade those out over time?
  Or do I have to save the lines to 10 different frame buffers and then fade those out?</p>
  
  <p>I'm not really sure where to start, sorry.</p>