Box objects (AABB) now have the long awaited extent() and center()
methods. Corners are now called “aa” and “bb” for convenience. The
WorldEntity class uses a box3 for its AABB instead of two vec3s.
This allows us to simplify a lot of verbose code.

scene so that g_scene can be deprecated later.

- fixed 1270 CR characters
 - fixed 56 trailing spaces
 - fixed 5085 tabs

reuse the linear operations in quaternions. Also mark some constructors
explicit to better spot coding errors.

sub-millisecond but can be improved if we get rid of SDL timers.