QTabletEvent::pressure() capped at ~0.474 under X11/xcb QPA, despite raw ABS_PRESSURE = 0–8191

This bug was originally reported on Krita's site.

Steps to Reproduce

1. Hook up XP Pen tablet and start driver

2. Run `evtest` to show the raw pressure output from the driver

  First identify the device by running `evtest`, and select the correct device

  Then check the values from that event.

# i.e
sudo evtest /dev/input/event25

3. Press on the tablet and watch the values ranges from 0-8191 as expected

4. Compile and run the attached `pressure_printer.cpp`

g++ -std=c++17 -fPIC pressure_printer.cpp -o pressure_printer \
    $(pkg-config --cflags --libs Qt5Widgets)

Run ./pressure_printer and then press your pen down on the white square that pops up.

Note the pressure value is maxed out at ~0.474

This can also be demonstrated by using the Tablet Tester in Krita (Settings > Configure Krita > Tablet Settings > Open Tablet Tester)

What should have happened

The value should have reached 1.0 as the pressure increased to the max value of 8191