harmonic example: Typo in light angle
quadtree: Fix Node initialization

Oops. This may have been causing bugs due to uninitialized children[] pointers
in leaves, but in practice I haven't seen this hit yet.
quadtree: Fix another intersection-test bug

First I thought this was a reflection bug, then it looked like a quadtree bug,
and finally it turned out to be another bad assumption in intersectAABB(). Oops.