Refactor insertion into BSP tree.

Before this commit, inserting into BSP tree could easily overflow
the stack because we allocate very large stack frames and, on
convex geometries (e.g. a sphere), the BSP tree degenerates into
a "BSP list", thus requiring one large stack frame per triangle.
This can be reproduced by exporting a 2d shaded view of sphere.

After this commit, the stack frames only contan a pointer to
a supplementary data structure, and moreover it only allocates
its fields on demand, conserving heap memory as well.

As a side effect, an arbitrary classifier limit of 50 vertices
is removed.