HTML5 backend doesn't support setMouseLock

[BSlug]

The HTML5 backend's function to lock the pointer is currently NOP.

Only calling the canvas field's requestPointerLock() method is insufficient. Rather than using a mousemove event's movementX/movementY fields to determine relative mouse movement, the backend currently uses cached clientX and clientY, which has the unfortunate side-effect of not working when the pointer is locked.

[joshtynjala]

I'm guessing that requestPointerLock() didn't exist at the time that mouse lock was originally implemented for the other targets that support it, so that's why it's NOOP for now.

It's probably not too difficult to switch to movementX/movementY once support is added.