Takes a 4-point cubic bezier curve as input and maps the pressure coordinates
to the values outlined by this curve. This is an extension of the current
implementation in the xf86-input-wacom driver which only allows the two center
control points to be modified.

Over the years a few users have noted that the wacom driver's pressure curve
makes it impossible to cap the pressure at a given value. Given our bezier
implementation here, it's effectively a freebie to add configurability of the
first and last control points. We do require all control points' x coordinates
to be in ascending order.

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