Head tracking in map mode doesn't play nicely w/ mouse drag

In map mode (and also in external mode, when using this patch #9), you can't drag the view around with the mouse while using head tracking. (If you attempt this, there is some visual studdering, and the view does not move.)

A good behavior would be to allow the user to drag the view around with the mouse and also be able to look around w/ head tracking at the same time.

I think this can be implemented w/ some extra state. The idea is, on each frame, to take the difference between the current and previous head tracking angles, and add that to the game's internal camera angle (which is updated based on the mouse and other factors). This may have problems with drift over time. Need to implement it and see if it works.