touchpad: use the same speed for scrolling as the baseline of the accel curve

Scrolling and gestures use unaccelerated motion. The idea behind it was that
at least for the default speed setting of 0, the accelerated speed and
unaccelerated speed are identical where meaningful.

The touchpad speed curve has a plateau for 'normal' speeds (i.e. not very slow
and not very fast) where the acceleration factor is constant. This is the
reference factor that the unaccelerated motion should use as well.

Since the touchpad acceleration rework in d6e53134 the reference factor is
0.9 * TP_MAGIC_SLOWDOWN (previously the factor was 1.0 * TP_MAGIC_SLOWDOWN)
and scroll motion is thus 10% faster than the pointer movement at the default
speeds. Let's fix this and let the two match up.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
(cherry picked from commit 2b33445b)