Rework positioning and scaling
It is necessary to be able to be able to position and scale the clock in a more sensible way.
When defining a clock face we should be able to specify its bounds in the current coordinate system. That is the area to be erased. It would be highly appropriate to be able to set clipping before erasing the canvas.
new ClockFace([], {position: [x, y], size:[w, h]})
This is still not ideal, but better than nothing. It is likely that "position" is unnecessary ("translate" probably gets the job done).
(A debug mode showing all the bounding boxes would be helpful.)