Short circuit a whole lot of proper motion, precession, nutation,
aberration calculations if the time change is insignificantly small
compared to the time-scales of these effects.

Potentially dangerous. I've already found one bug in my code which I'm
going to fix right away.

The two primary factors leading to slow drawing are the drawing itself
and the JIT Update on DeepStarComponent. While using OpenGL rendering,
the former bottleneck is not as dominant as the latter one. This is
expected to make significant differences to FPS in OpenGL mode. In
QPainter mode, there were noticable differences seen using a profiler,
but very little visible difference (although there was visual evidence
of some improvement).