Use the helper function for mapping a stylus tool evdev code to a
clutter button code. This fixes a (theoretical) issue - if a tool were
to send any button other than the one we handled those would likely be
BTN_SIDE and friends and we'd likely end up with negative button
numbers. The BTN_TOOL_PEN range is not predicable enough to do any sort
of calculation conversion because things like BTN_TOOL_DOUBLETAP have
specific meanings that aren't actually buttons.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3649>
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