notify_touch_normalized() is an extended form of notify_touch(), adding
normalized touch coordinates which are necessary for calibrating a
touchscreen.

It would be possible to invert the transformation and convert from
global coordinates to normalized device coordinates in input.c without
adding this API, but this way it is more robust against code changes.

Recovering normalized device coordinates is necessary because libinput
calibration matrix must be given in normalized units, and it would be
difficult to compute otherwise. Libinput API does not offer normalized
coordinates directly either, but those can be fetched by pretending the
output resolution is 1x1.

Anticipating touch calibration mode, the old notify_touch() is renamed
into a private process_touch_normal(), and the new
notify_touch_normalized() delegates to it.

Co-developed by Louis-Francis and Pekka.

v2:
- introduce struct weston_point2d_device_normalized
- rename notify_touch_cal() to notify_touch_normalized()
- remove WESTON_INVALID_TOUCH_COORDINATE

Cc: Louis-Francis Ratté-Boulianne <lfrb@collabora.com>
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
v1 Tested-by: Matt Hoosier <matt.hoosier@gmail.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>