Fix "Quick Undo"

[1j01]

When you're drawing a shape or stroke and you press the opposite mouse button, i.e. L+R together, it's supposed to immediately cancel the drawing operation. For touch, it should cancel if you touch with a second finger.

I actually had it working using the same code for mouse and touch, listening for both interactions as a secondary pointerdown. But that's not how pointer events work anymore for mouse presses. Either Chrome updated or I updated PEP and now a mouse will only have one "pointer" down at a time, but will get pointermove events (which ought to be called pointerchange) with button/buttons.

I tried to reimplement this, but while it fixed it on Chrome, it broke it on Firefox.