Fix freehand-drawn sliders with distance snap

[default0]

Trying to freehand draw a slider when distance snap is active causes the control points to be offset from the slider curve.

The following video demonstrates the current behaviour on master:

2023-11-22.09-52-22.mkv.mp4

There are two possible ways to fix it:

• We can insert the linear point (0, 0) into the bspline builder to trace a path from the hitobjects snapped position to the current cursor position, resulting in this behaviour:

2023-11-22.09-51-54.mkv.mp4

• Or we can reposition the hit object so it ignores distance snap and instead starts at the current cursors location

2023-11-22.09-54-08.mkv.mp4

Given that freehand drawing in combination with snap feels strange, another option would be to simply disable the option to freehand draw (ie returning false from OnDragStart) if the hitobjects position is sufficiently far away from the cursor position, however, I fear this might be unintuitive to users.

This PR implements repositioning of the slider to the cursors current location when freehand drawing starts, because to me this seems like the most intuitive approach.

[peppy]

I think I prefer the case that respects distance snap personally..

[default0]

Updated :-)

[Tom94]

I've got a third proposal: what about translating the generated path by the offset to the snapped position on drag start? That way, the user's cursor wouldn't coincide with the endpoint of the path anymore, but there wouldn't be a weird linear segment at the start and distance snap would still be respected. EDIT: might have to hide & freeze the actual mouse cursor while drawing is ongoing to make this proposal ergonomic

[peppy]

Hiding the mouse sounds very weird..

[bdach]

I'm just going to get this in, in its current state, to fix the clearly broken behaviour for next release. We can iterate on this going forward based on user feedback.