Don't assume tablet (x,y) always starts from (0,0)

Newer Cintiqs have an outbound of 200 tablet count outside of screen
area. This causes absinfo.minimum to be 200 for some tablets instead
of always 0 for all tablets. Update struct _WacomDeviceRec and
_WacomCommonRec to process this change.

Signed-off-by: Ping Cheng <pingc@wacom.com>
Reviewed-by: Jason Gerecke <killertofu@gmail.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>