Tools on the same physical item have the same serial number, so the eraser
and the pen part of a single pen share that serial number. With the current
lookup code, we'll always return whichever tool comes first into proximity.
Change the code to use the hw id in addition to the serial number, this way we
can differ between two tools.
gdk_seat_default_grab() grabs POINTER_EVENTS if the capability is
GDK_SEAT_CAPABILITY_ALL_POINTING. But that enumerator is a union that
includes GDK_SEAT_CAPABILITY_TOUCH, but we never grabbed TOUCH_EVENTS,
an unused macro that was presumably created with this purpose in mind.
So, check which of the ALL_POINTING capabilities we have, and set the
right mask of POINTER_EVENTS and/or TOUCH_EVENTS as required.
As part of this, explicitly let TABLET_STYLUS take over pointer events,
as this is the intended behaviour and was the effective result before.
This should fix touch events being lost in migrating from Device.grab()
to Seat.grab(GDK_SEAT_CAPABILITY_ALL_POINTING), as found by Inkscape.
https://bugzilla.gnome.org/show_bug.cgi?id=781757
