Enable pen/stylus erasing events

[mccoykillian]

Currently pointer-tracker only accepts basic contact events for mouse, touch, and pen (button state 0:

(from https://developer.mozilla.org/en-US/docs/Web/API/Pointer_events)

The button and buttons state needed to detect pen erasing is 5 and 32 respectively, but pointers with these states are blocked in the _pointerStart function:

Could this function be updated to support other button states or at least the pen erasing combo?