exitPointerLock workaround to unblock Safari / Mobile Safari

Hi! I've only had a chance to test this on Safari 6.0.4 and the Mobile Safari bundled with iOS 6.1.2. Apologies if this is already being tracked - I couldn't find any references to it.

Without this change, Safari appears to hit a certain (shallow) point and then silently errors out with:

    TypeError: 'undefined' is not an object (evaluating 'canvas.exitPointerLock.bind')

which is raised by this line:

    canvas.exitPointerLock = canvas.exitPointerLock.bind(document);

So the change simply tests the existence of canvas.exitPointerLock prior to calling and skips the call if the test fails.