Incorrect box-shadow rendering when the blur of adjacent rounded corners overlaps

[mstange]

https://bugzilla.mozilla.org/show_bug.cgi?id=1423692

Testcase

Actual vs expected results:

---
root:
  items:
    - type: stacking-context
      bounds: [0, 0, 1000, 1000]
      items:
        - type: box-shadow
          bounds: [ 50, 10, 200, 80 ]
          color: black
          clip-mode: outset
          offset: 0 150
          blur-radius: 20
          border-radius:
            top-left: [100, 40]
            top-right: [0, 0]
            bottom-left: [100, 40]
            bottom-right: [0, 0]

[mstange]

I thought about this problem some more and came to a surprising conclusion: Blurring corners separately and combining them afterwards into the whole blurred box works fine at all sizes, even if the box is so small that the blur from one corner extends into the blur from another corner! In the overlapping part, the correct blurred result can be computed from the individually-blurred corners like this:

float shadow_alpha(pos) {
  return sample_from_one_blurred_corner(pos)
          + sample_from_the_other_blurred_corner(pos)
          - sample_from_the_repeated_middle_part(pos);
}

Proof via picture:

This is even true more generically:

It looks like this works because of the distributive property of discrete 2d convolutions.