tablet: scale the available pressure range into the pressure thresholds

tablet: scale the available pressure range into the pressure thresholds

Pens that don't have a pressure offset (caused by a worn-out tip) still have
basic pressure thresholds to avoid tip events when we're still a bit away from
the tablet or barely touching it. That range is currently 5% of the pressure
for tip down, 1% for tip up.

This leaves us with 95% of the range and that needs to be scaled correctly,
otherwise the bottom 5% happen before a tip event and are inaccessible where
applications don't look at pressure before tip down.

Fixes #332

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>