filter: add a trackpoint multiplier factor

filter: add a trackpoint multiplier factor

Measuring the trackpoint range has not shown to be sufficient or precise
enough to be used as an ingredient for trackpoint acceleration. So let's just
switch back to a generic multiplier that we can apply to the input deltas do
undo any device-specific lack of scaling.

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