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>