There are three cases:
(1) no pressure supported (i.e. your mouse moving around in the vnc
window and qemu reporting this as tablet coordinates).
(2) just pen/finger present/not present supported. pressure jumps
between 0 and max (and we can make max == 1 in that case).
Phew - that's one of the bits where touchpads and tablets behave
differently IIUC. For touchpads, pressing means movement happens.
For tablets, pressing means an actual press, as if you would press
your pencil on a sheet of paper. So for tablets, pressure==1
basically means click.
Now the thing is that multitouch gestures might take non-click
pressure into account for their calculations. So we need pressure
for movements on touchpads, but with tablets, we only need pressure
when clicks happen. For everyone else, clicks would just be a button
press.