title: force html canvas to draw only on a certain area for some drawings
body: <p>My objective is this :
  creating a rolling text that does not cross borders of a rectangle inside the canvas.
  Some text should appear partially. Yet i have not found a way to obtain this. </p>
  
  <p>The only thing i can think is of some setting of the canvas like rotate, and translate that show the drawings only in the limited area, as long as i don't reset the state of the canvas.</p>
  
  <p>I uploaded an example of what the behaviour should be where the text is rolling from right to left (note the 'g' is cut as desired) :</p>
  
  <p>!!! Sorry , the admin don't let me to upload images yet , see this at 
  <a href="http://jsdefense.sourceforge.net/rpsz/ex_rolling_text.jpg" rel="nofollow">http://jsdefense.sourceforge.net/rpsz/ex_rolling_text.jpg</a></p>
  
  <p>Another solution could be to print text over a brand new Image and slice it when drawing ?</p>