This tablets gets a lot of things wrong:
- the secondary button is reported through Secondary Tip Switch
- the third button is reported through Invert

Fortunately, before entering eraser mode, (so Invert = 1),
the tablet always sends an out-of-proximity event.
So we can detect that single event and:
- if there was none but the invert bit was toggled: this is the
  third button
- if there was this out-of-proximity event, we are entering
  eraser mode, and we will until the next out-of-proximity.

Link: https://lore.kernel.org/r/20240410-bpf_sources-v1-8-a8bf16033ef8@kernel.orgReviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>