Automatically generate quadtree children.

This mostly works, and does the view reparenting. It doesn't do any clipping,
so we still hit precision issues as (for example) bézier control points are
outside the quadtree node's coordinate system, and become very, very large.

There are some issues with the boundaries of quadtree nodes, as the system
currently can't display two nodes at once. To compensate, it traverses up the
tree until it finds a single node which contains both, though this will be
a distant anscestor if the nodes are distantly related.

Quadtrees are still disabled by default. There are also a couple of minor
changes to the GL code to make it spit out fewer error messages and run
slightly faster on some hardware.