add intersect-plane-sphere? fn
bugfix intersect-plane-sphere? and add to intersect-shape impls for Plane/Sphere
refactor add-point* to avoid SO
csg ns refactor

add CSGNode defrecord
replace multi-key assoc's w/ CSGNode ctors
replace laziness in csg-node w/ calls to `into`
update mesh->csg to pre-compute CSGNode to improve performance for successive csg ops
update union/subtract/intersect to not serialize result node, now only done in csg->mesh fn
update csg->mesh to use BasicMesh (instead of GMesh)
re-add (fix) bounding sphere culling