Looks like with some versions of Xlib (at least the 1.6.2 currently used on Debian testing)
and/or evdev generic driver (2.9.0 currently on Debian testing), you have to also 'select'
DeviceButton1Motion with the extended tablet's motion event, otherwise you won't get any
tablet motion event once pen is pressed, leading to no pressure (each stroke keeping its
init pressure until the end). Crap!
0d5ebabe43