Previously the slider path length would be snapped using the current
beat snap setting on *every* change of the slider path. As it turns out
this is unexpected behaviour in some situations (e.g. when reversing a
path, which is expected to preserve the previous duration, even though
the slider may be technically "unsnapped" at that point in time due to a
different beat snap setting being selected afterwards).
More specifically, if the left mouse button was just pressed without a
drag, `OnDragEnd()` wouldn't fire, and the next drag would start moving
the last placed control point around regardless of where the mouse was.