Added clearing method.
Big revision.

* Found better way to deal with flonum inaccuracies: slight
  reassociation of operations seems to keep things under control much
  longer.

* Put bound on redraw time; if we exceed it, stop trying to refine.
  (This avoids the UI prompt from firefox about a script taking too
  long.)  I should remove this if I, as planned, move this all into a
  background web workers task.

* Completely revised itercountToColor; I think this hits a nice
  sweet spot for seeing a "wow" when the iteration count hits
  certain thresholds and colors pop out.

* Stopped rebuilding color map on each itercount adjustment; just
  build a single large one once, and take iterations modulo its
  length.  (That's good enough for most purposes)

* Split the iteration step into two parts when rebuilding the color
  map takes too long.  (Heh, I guess I should take this out in light
  of the previous bullet.)

* Fixed some bugs where the system was not resuming the iterative
  refinement in response to mouse clicks (which could indeed bring the
  focus to a place where we might make more progress, especially if a
  large percentage of the canvas has itercount = 0.