Closes https://github.com/ppy/osu/issues/10672.
In two minds about how this should be implemented but went in this
direction initially. The other way would be to add local handling of
Shift-Right Click inside PathControlPointPiece (which is already doing
mouse handling itself).
Using an abstract property was awkward for this as it is being consumed
in the underlying constructor but could not be dynamically set in time from a
derived class.
This results in slider placement including "excess" length, where the
curve is not applied to the placed path. This is generally not what we
want.
I considered adding a bool parameter (or enum) to change the
floor/rounding mode, but on further examination I think this is what we
always expect from this function.