[css-shapes-2] In shape() It should be possible to specify control points relative to their point on the curve #10667
Description
In CSS Basic Shapes 2, the [shape()](https://drafts.csswg.org/css-shapes-2/#shape-function) function allows you specify points with to or by which indicates whether the point, and its control points, are specified in "absolute" terms relative to the bounding box, or in "relative" terms from the current path point.
However, in many cases I think authors will want to specify control points relative to the path point that they are attached to. I should be able to say something like curve to 120px 140px with -12px -10px / 15px 20px to say that the first control point is at -12px -10px relative to the start of the segment, and the second one is at 15px 20px relative to the end point.
We'd need some reasonable behavior for quadratic segments, and smooth segments.