drawKnob()
Calculates the rectangle in which the knob should be drawn, then calls Drawknob(_:) to actually draw the knob.
Declaration
func drawKnob()Discussion
Before this message is sent, a lockFocus method must be sent to the cell’s control view.
You might call this method if you override one of the display methods belonging to NSControl or NSCell.
Special Considerations
If you create a subclass of NSSliderCell, don’t override this method. Override drawKnob(_:) instead.