By default, the X server maps the tablet axes to the available screen area.
When a tablet is mapped to the screen but has a different aspect ratio than
the screen, input data is skewed. Expose an area ratio property to map the
a subsection of the available tablet area into the desired ratio.

Differences to the wacom driver: there the x/y min/max values must be
specified manually and in device coordinates. For this driver we merely
provide the area ratio (e.g. 4:3) and let the driver work out the rest.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Jason Gerecke <jason.gerecke@wacom.com>