With the great help of Magnus Vigerlof, plenty of emails back and forth to understand it, and finally Pippa and Ben Avery who lent me their wacom tablet over the weekend I'm happy to announce that tablet support (i.e. scaling of input coordinates) is back. This was the last big chunk missing.

The coordinate handling to the client is:
rootx/y and eventx/y of a DeviceXXXEvent are now always in screen coordinates. This eases the development of XI clients that don't really care about device coordinates.

valuators0...32 are always in device coordinates, and the coordinates depend on the device mode. If the device is mode relative, the valuators are relative, if the device is mode absolute, the coordinates are absolute.

One thing that we need to think about is whether we introduce a ReportingMode in addition to the device mode. ATM, it is not possible to have a device report relative coordinates if it is in absolute mode.