Layout: stylistic changes

Additionally if an image border can't be displayed show solid border.
Introduce build_display_list_for_border_image to display border images.

Upstreamed from servo/servo#21608 [ci skip]

css/css-backgrounds/border-image-width-008.html

@@ -2,7 +2,7 @@
 <meta charset="utf-8">
 <title>border-image-width has the same effect as a border-width and the image is displayed even if border-width is zero</title>
 <link rel="match" href="border-image-width-008-ref.html">
-<link rel="help" href="https://drafts.csswg.org/css-backgrounds-3/#propdef-border-image-width" />
+<link rel="help" href="https://drafts.csswg.org/css-backgrounds-3/#propdef-border-image-width">
 <style>
     #test {
         width: 400px;