Wacom ArtPen requires pressure interpolation

Originally filed as xf86-input-wacom issue #336

[...] the Wacom Art Pen (KP701E2) only reports the pressure data every other response. This is not an isolated issue to a single pen. Have tested this with multiple art pens. I've also talked to others with Art Pens and when I get them to disable the high sensor stabilizers they have set in (painting) software, the pen pressure stepping also appears instantly.

Recordings from that bug show this is a hw limitation: libinput-record output and hid-record output. The hidraw output for pressure only changes ever second event and the events are missing from every second evdev event as the kernel optimises them out.