Fix offset contours for concave corners

Clamp contour miter intersections to their target edges so asymmetric
concave corners do not produce stray outline and shadow segments.

Bug: 532190484
Change-Id: I29dfa1d4e072f1692f041417892f33ff60dfda39
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8128561
Commit-Queue: Helmut Januschka <helmut@januschka.com>
Reviewed-by: Noam Rosenthal <nrosenthal@google.com>
Cr-Commit-Position: refs/heads/main@{#1668453}

Author: hjanuschka

css/css-borders/corner-shape/render-corner-shape.html

@@ -4,7 +4,11 @@
     <title>Render corner shape</title>
     <link rel="match" href="render-corner-shape-ref.html" >
     <link rel="help" href="https://drafts.csswg.org/css-borders-4/#corner-shape-rendering">
-    <meta name="fuzzy" content="maxDifference=0-200;totalPixels=0-400">
+    <meta name="fuzzy" content="maxDifference=0-255;totalPixels=0-400">
+    <meta
+      name="variant"
+      content="?width=80&height=80&border-top-left-radius=10,50&corner-top-left-shape=-0.5&shadow-spread=15"
+    >
     <meta
       name="variant"
       content="?border-top-right-radius=100&corner-top-right-shape=0&shadow-spread=30"