Fetch tablet mapping from Wintab instead virtual screen geometry

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