progress. I want to change the sketch.h stuff fairly significantly,
I think, before proceeding.
[git-p4: depot-paths = "//depot/solvespace/": change = 1657]
draw planes that I will use to do the datum planes. Still haven't
settled upon the request/entity/group structure, and that's the
dependency before doing much real.
[git-p4: depot-paths = "//depot/solvespace/": change = 1656]