title: getUserMedia lock focus/exposure
body: <p>I am using <code>navigator.getUserMedia</code> with constraints to access the user's webcam, using the feed as the source of an HTML <code>&lt;video&gt;</code> and then copying its stream to <code>drawImage</code> a <code>&lt;canvas&gt;</code> context. I'm doing all this so I can take a snapshot at intervals.</p>
  
  <p>What I would like to do is, once the page starts taking snapshots, lock the getUserMedia camera's focus/exposure, so that in between snapshot intervals the environment can change without the light balance changing or the camera refocusing.</p>
  
  <p>Does anyone know if this is possible on the JS side?</p>