tablet: ignore movements started outside the configured area

tablet: ignore movements started outside the configured area

If a tablet has an area configured and the pen goes into proximity
outside this area, ignore all events from this sequence. This truly
deactivates that area so it can even be used for e.g. placing a pen
there.

For simplicity, a sequence that starts outside the configured area will
be completely ignored, i.e. moving into the tablet area will not trigger
any fake proximity events as we cross into the allowed area. This
requires quite a bit of effort and it's unclear if it's really needed by
users - we can reconsider when we get complaints.

We do however accept a proximity event within within 3% of the
configured area.  This gives us 6mm on a 200mm tablet where we can move
in from the area and still have events work, i.e. some error margin for
where a user needs both an area and work closes to the edge of that
area.

Part-of: <https://gitlab.freedesktop.org/libinput/libinput/-/merge_requests/1013>