Faster, more precise detection of separating hyperplanes in BSP tree build

The new method finds bounding planes for polygons using their
(approximate) normals and works for any number of them instead of
just two. The algorithm is O(n^2) where n is the number of polygons,
so it is currently applied only when n <= 5; i.e. near the leaves.
BSP tree type simplified; added support for no-polygon scenes

Previously, the BSP type had a "negative" and "positive" subtree, and
a collection of unsorted "zero" (on the plane) shapes. Rather than "zero"
being a subtree (which would sort the shapes), it's rolled into "positive",
with corresponding changes to the BSP walk.

BSP tree build now supports scenes without polygons, though it does split
on polygonal planes first. When all such splits fail (usually because there
are no polygons, but might be because all the shapes lie on one polygon's
plane), it tries splitting on planes containing line segments. When that
fails (usually because there are no line segments), it tries splitting
disjointly on all remaining vertexes, then splitting at the midpoint of
the longest axis.

Upshot: it'll draw a double helix created by two `parametric3d' renderers
properly.
Reduce point3d rendering time by 50-70% by keeping points grouped

The old depth sort hack split up groups of identical points in order
to sort them. Now the BSP tree build keeps them grouped, only splitting
groups of points as needed. Drawing parameters like color and pen
width are set only once for each group instead of once for each point.
The speedup is from reducing the number of calls to set them.