How to delegate the mouse down event?

Question

JavaScript

Hello dear developers.
I implemented this thing https://jsfiddle.net/lokdmt/cjj6b5x7/8/ , where you can erase dirt from a T-shirt and after a certain percentage of erased stains a popup appears (now it's just an alert), but that's not the point.
The task is as follows: the client grabs a sponge with the mouse and erases the stains.
At the moment, the spots are erased separately with the mouse cursor, and you can grab the sponge separately. But with a captured sponge, the stains are not erased.
I suspect that it is possible to delegate the event of the pressed mouse to the sponge, but it does not work.
I would be very grateful for help or advice on how to implement this better.

Answer 1

Why delegate?
Is there a delta of mouse movements? When grabbing the sponge, you begin to transfer the movement to the sponge and
the PS mask, the comma should be removed from the reference)