[css-borders-4] corner-shape should impact the rendering of disjoint color corner joins

[smfr]

I think corner-shape: notch should affect how the join between two colors is rendered.

Here's a rendering of

        .box {
            width: 100px;
            height: 100px;
            box-sizing: border-box;
            background-color: silver;
            border-width: 20px;
            border-color: green;
            border-style: solid;
            corner-shape: notch;
            border-right-width: 0;
            border-bottom-width: 0;
            border-top-left-radius: 34%;
            border-left-color: blue;
        }

Now if we make the left border thinner, we get:

This looks wrong. I think the color join should happen along the intersection of the two inner corners of the notch.

With other corner shapes, the corner join that falls out of existing code (a line intersecting the border box and padding box corners) seems OK (excuse the glitchy rendering):

[tabatkins]

Sounds reasonable, tho note that "notch" is just the result of an infinitely negative value for superellipse(), so it should produce this behavior as a smooth limit. Perhaps it can start transitioning from the "default" position to the "notch" position starting at the inverse squircle value (-2) and ending at, say, -10?

[noamr]

The current CSS spec doesn't specify color joins in an interoperable way. Do we want to change this here?