I think that the appropriate proposal should not be to make the pinch gesture
do any specific thing, but to treat it in the same way that other events like
scrollwheel and mouse events are treated.  Namely, it should have a name and
it should be possible to bind the event to a procedure.

I don't know how it works.  Since you can continuously zoom and unzoom with
that gesture, I assume that many events are sent, as when the mouse is moved.
I would guess that the event provides some numerical value indicating how much
closer or further apart the fingers moved since the previous event.  And maybe
events are sent for the start and end of the gesture. But this is all guesswork.

I also don't know which platforms support the gesture nor how they support it
if they do.  So lots of research and experimentation would be needed.