title: Defining how 2 separate, overlapping canvases blend
body: <p>I basically want to put a canvas on top of another and define the way their contents are blended.</p>
  
  <p>I have one white canvas with black characters on it, and I want to highlight a part of it with a transparent blue rectangle, without having my black characters in the background turning dark blue. In fact, <strong>I need the aspect I'd get if I merged the two canvases with globalCompositeOperation set to "multiply" instead of default, while keeping both canvases separated and overlapping.</strong></p>
  
  <p>Here's what I have :<br>
  <img src="https://photos-6.dropbox.com/t/0/AADolr88uN5uvuDhY3-wVzlQcaAhVzkD3BmIhDdNmtC4WQ/12/147137725/png/1024x768/3/1398265200/0/2/whatIHave.png/zbyoL6NJDK3f-mDkTWR5vvGrjqrQ-B9No3iDXF1TAd4" alt="https://www.dropbox.com/s/zw0h9yjxsk4z7ug/whatIHave.png"></p>
  
  <p>Here's what I want :<br>
  <img src="https://photos-3.dropbox.com/t/0/AABDdx068CO2mIchi-pcj3ttRhx-UHD09J_frfFX265udw/12/147137725/png/1024x768/3/1398265200/0/2/whatIWant.png/LsbO9qagn3rbkpzg1R6caSq0yPX2gpf6JgSLSHEt2u4" alt="https://www.dropbox.com/s/r3nrb9lxx03xbjh/whatIWant.png"></p>
  
  <p>I am aware that globalCompositeOperation would allow me to do that if I merged the two canvases into one. But I'd rather keep both canvases : my background canvas is displayed by a lib. I can still draw in it, but that would complicate things a lot:</p>
  
  <ul>
  <li>I'd be too dependent on their logic and would have to tweak mine and theirs to make it work,</li>
  <li>performance is critical and this solution would imply much more drawing at 24fps,</li>
  <li>I'd struggle every time the lib is updated...</li>
  </ul>
  
  <p>All in all it seems way better to keep away from interfering with the lib. Is there a way to choose how overlapping canvases will behave?</p>
  
  <p>Thanks in advance!</p>
  
  <p><strong>EDIT:</strong> We've also thought of transforming the white parts of the background canvas into transparent parts and adding our highlight canvas underneath, but that's also complicated, if not impossible.</p>