Pass on Wacom pad keys
Wacom Cintiq 24HD send
KEY_PROG1 through to
KEY_PROG3. More recent tablets send
KEY_BUTTONCONFIG. These events are not passed on by libinput.
We need to pass these events on, either as
LIBINPUT_EVENT_KEYBOARD_KEY or as new
LIBINPUT_EVENT_TABLET_PAD_KEY. The latter is better since it's maintains the distinction between pads and keyboards but requires a new interface. We cannot re-use
LIBINPUT_EVENT_TABLET_PAD_BUTTON because those are sequentially numbered whereas the keys have a specific, hardware-associated meaning.
Most of the Wacom Cintiq since the 24HD series, I think.