Fix a bug in BBox (introduced by the RRay changes).
Make the app fullscreen in interactive mode.

Non-interactive renders remain in a window as before.
Interactive renders occupy the full screen (switching screen resolution if
required) by default. This can be switched off by specifying
"fullscreen false" in GlobalSettings.
Inline the fresnel function, as this improves performance about 1-2%.
A significant overhaul of the interactive mode.

- take out the keyboard handling outside of mainloop() to keep it short and
  simple.
- take some keys as key click events (e.g., ESC). Movement keys continue
  to be sampled with GetKeyState.
- handle mouse motions as well (mouse looks around)
- the last-frame rendertime is used to determine the sensitivity of the
  keyboard controls (i.e. movement in world units is the same speed,
  regardless of the FPS count)
- The key R toggles "running mode", where the camera motion is 4 times
  faster
- The key P prints out camera position and orientation (for scene
  editing and camera placement).
- Take out sensitivity adjustments outside Camera::move() and
  Camera::rotate().
Target this scene for interactive mode.