title: Two canvas above each others return different x,y coordination
body: <p>I draw 2 canvas above each other,in bottom I loaded an image.
  I'm using the top canvas for mouse move events and draw some info according to the image loaded on the bottom canvas (for example, when mouse above eyes, I draw circle).
  The problem is that eventhough both canvas has the same width &amp; height, the coordination of x,y of the top canvas seems not fit the bottom one, do I missing anything?
  I tried to set the top canvas.scale as the one bellow after loaded the image.
  Thanks!
  JB</p>