Distance between the last drawn point and the current point is
calculated to be able to discard very short stroke segments (i.e we only
add to the current stroke if the input device has moved more than a
certain distance since the last drawn point).

This commit moves this code from the stroke to the scene, which allows
to calculate distance more accurately: it is now calculated as the
total, absolute distance traveled since the last point, rather than simply the
length of a line between the last point and current one.