title: clear line on HTML5 Canvas
body: <p>Is there a way to unstroke a line?
  On my canvas I have a line with opacity 0.5, and width of 20 pixels, let's say.
  Now I want to to make it longer, means to draw another line right out of the old one.
  when doing that, the matching points between the old and the new lines become less transparent (because they're made of two lines now).
  so I want to unstroke the old line and then stroke the new one.</p>
  
  <p>how can I do it?</p>
  
  <p>thanks</p>