Canvas: clear rect but keep the rest of the content
<p>I want to draw some text onto my canvas and want to clear everything which was drawn out of a specific area (area defined by a Rect). </p>

<p><img src="http://i.stack.imgur.com/47X6p.png" alt="enter image description here"></p>

<p>I draw the magenta-Text and want to clear everything in the grey and keep everything in the white box. I got the white area as a rect.</p>
