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.
This is quite a breaking change, but I think it is beneficial due to the large amount of usage of this class.
I originally intended just to remove the allocations of the two delegates handling the `Changed` flow internally, but as nothing was really using the bindables for anything more than a general "point has changed" case, this felt like a better direction.
`SliderSelectionBlueprint.OnDeselected()` would expire the
`ControlPointVisualiser` on deselection, leading to its removal from the
blueprint and eventual disposal, but still kept a separate reference to
said visualiser in another field.
This could lead to that stale reference to a disposed child getting read
in `ReceivePositionalInputAt()`, crashing quite a ways down over at the
framework side on futilely trying to compute the bounding box of a
drawable with no parent.
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).
