Altering the shape of a default slur with a list of offsets

If you want to adjust the shape of a slur, one way is to specify a new list of control-points. This function offers a different approach by allowing you to adjust existing control-points. This is done by entering a list of offsets (in staff spaces) to the default coordinates:

\shapeSlur #'(x1 y1 x2 y2 x3 y3 x4 y4)

Setting a value to 0 leaves the default coordinate unchanged, so it is easy to maintain a slur's attachment-points while changing its interior.