tablet: add tilt-based touch arbitration for screen tablets

tablet: add tilt-based touch arbitration for screen tablets

If the tilt angle on tip down is not 0 set the touch arbitration to a
rectangle around the assumed position of the hand. This assumed position is
right of the tip for a rightwards tilt and left of the tip for a leftwards
tilt (i.e. left-handed mode). The rectangle is 200x200mm with a 20x50mm
NW of the tip or NE for left-handed. In other words, if the period below is
the tip, the rectangle looks like this:

    +-----------+                          +-----------+
    | . 	| <- for rightwards tilt   |         . |
    |           |                          |           |
    |           |                          |           |
    |           |    for leftwards tilt -> |           |
    +-----------+                          +-----------+

Touches within that rectangle are canceled, new touches are ignored. As the
tip moves around the rectangle is updated but touches are only cancelled on
the original tip down. While the tip is down, new touches are ignored in the
exclusion area but pre-existing touches are not cancelled.

This is currently only implemented in the fallback interface, i.e. it will
only work for Cintiqs.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>