Some devices, like Microsoft Surface Pro 5, don't map tablet's
input range to the entire virtual screen area, but map it to
the primary display that has actual built-in tablet sensor.

In such cases we should fetch actualy mapped aread from Wintab's
lcSys{Org,Ext}{X,Y} fields and use it for cursor mapping.

The patch also introduces an environment variable switch that
falls back to the old method of mapping:

QT_IGNORE_WINTAB_MAPPING=1

When the variable is set, the scaling is done via virtual desktop
area only.

BUG:406520