dix: Add valuator_mask_set_absolute_unaccelerated

Add a valuator mask setter for setting absolute coordinate combined
with unaccelerated motion deltas. This will later be used by Xwayland
to combine a wl_pointer.motion() event with the unaccelerated delta of
a wp_relative_pointer.relative_motion() event.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>