wacom: Drop old_axes from calibration API

The calibration utility was modified in cf408c27 to return unitless
padding measurements instead of axis values for storage in gsettings.
Unfortunately, the code still assumes in some places that it is working
with axes rather than paddings. This causes subtle math errors that
result in undesired cursor offsets after the calibration is applied.

Fortunately, this can be simplified, since tablet area is always reset
to the default state before starting calibration, we are sure that the
value will remain constant. Since both axes are in the same 0..1 scale,
calibration code doesn't need to swap X/Y back and forth to calculate
each axis scale.

Additionally, the code to get the calibrated axis values has been moved
into its own function along with a new function that returns padding
values suitable for consumption by g-c-c. All calculations are performed
internally in the 0..1 range.

https://bugzilla.gnome.org/show_bug.cgi?id=784009



Co-Authored-By: Carlos Garnacho <carlosg@gnome.org>