onUpdate() called on tap despite no image changes
repro:
1. display a Sketch
2. tap the canvas (do not drag)

expected: 
Either 1) a point drawn on the screen and onUpdate() called (preferred), or 2) (based on the explanation below) no point drawn on the screen and onUpdate() not called.

actual: 
No point drawn on the screen and onUpdate() called.

description:
From the linked docs on smooth freehand drawing at [https://code.tutsplus.com/tutorials/smooth-freehand-drawing-on-ios--mobile-13164](https://code.tutsplus.com/tutorials/smooth-freehand-drawing-on-ios--mobile-13164):

> Another case I neglected is handling a touch sequence that consists of the user touching the view with their finger and then lifting it without having moved it -effectively a tap on the screen. The user would probably expect to draw a point or small squiggle on the view this way, but with our current implementation nothing happens because our drawing code doesn't kick in unless our view receives the -touchesMoved:WithEvent: message.

Given that I'd expect onUpdate() to not be called on a tap -- but it is. 