tablet: increase pressure offset limit from 20% to 50%

tablet: increase pressure offset limit from 20% to 50%

detect_pressure_offset() currently rejects offsets that are greater than
20%. My graphics tablet (Wacom Bamboo Fun) is about 30%. The pen tip is
2 mm. Wacom recommends replacing at 1 mm, which means this isn't worn
out yet and we should instead increase the limit to make these devices
usable.

Without this change a "pen down" event happens simultaneously with the
pen being detected -- about 1 cm above the surface -- and producing
libinput pressure of about 0.30. This means you start drawing "in the
air", without knowing up front where the cursor is going to be.

Signed-off-by: Bjørn Forsman <bjorn.forsman@gmail.com>