title: clearing circular regions from HTML5 Canvas
body: <p>It appears the only way to clear a region from a canvas is to use the clearRect() command - I need to clear a circle (I am masking out areas from a filled canvas, point lights in this specific case) and despite all attempts it does not seem possible.  </p>
  
  <p>I tried drawing a circle with an alpha value of 0 but simply nothing would appear unless the alpha was higher (which is counter to the point :P) - I assume because a contex.fill() draws it as an add rather than a replace.</p>
  
  <p>Any suggestions on how I might be able to (quickly) clear circles for mask purposes?</p>