Changes

# <strike>Reset the mouse position back to the original position when unlocking.</strike> - humph

# "When unlocked, the system cursor can exit and re-enter the user agent window. If it does so and the user agent was not the target of operating system mouse move events then the most recent mouse position will be unknown to the user agent and movementX/Y can not be computed and must be set to zero" diogogmt

# Before locking the mouse check if the element is a DOM element and if it is in the DOM Tree diogogmt

# Fix accurateness of mouse positioning on unlock() (should be the same point as when lock() was called). Currently works, but is offset. See nsEventStateManager::SetMouseLock. - JSilver999