We need a Wayland pen driver for the Fl::Pen API.

See FL/core/pen_events.H, src/drivers/Stubs/Fl_Stubs_Pen_Events.cxx, src/drivers/Cocoa/Fl_Cocoa_Pen_Events.mm

Resources:

1. Low-level: evdev, /dev/input/event*, libevdev,
   https://www.kernel.org/doc/html/latest/input/event-codes.html
2. Mid-level: XInput2 (for X11), XI_Motion, XI_ButtonPress
   https://www.x.org/releases/current/doc/inputproto/XI2proto.txt
   https://www.freedesktop.org/wiki/Software/libevdev/
3. Mid-level: Wayland tablet protocol, tablet-v2 protocol,
   zwp_tablet_tool_v2_listener, zwp_tablet_v2, zwp_tablet_seat_v2
   https://wayland.app/protocols/tablet-v2

https://github.com/Wacom-Developer/wacom-device-kit-linux/blob/master/Wayland/SampleCode/client.c
https://github.com/Wacom-Developer/wacom-device-kit-linux