Extend slider selection box bounds to contain all control points inside

[bdach]

Previously, the selection box was only guaranteed to contain the actual body of the slider itself, the control point nodes were allowed to exit it. This lead to a lot of weird interactions with the selection box controls (rotation/drag handles, also the buttons under/over it) as the slider anchors could overlap with them.

To bypass this issue entirely just ensure that the selection box's size does include the control point nodes at all times.

before	after

[OliBomby]

I was concerned that this would push the selection box controls offscreen but in practice this happens very rarely and even if it happens, you can use the rotation and scale tools in the transform toolbox. This is a good change.