layout: Add initial support for bidirectional text (BiDi) (#33148)

This adds supports for right-to-left text assigning bidi levels to all
line items when necessary. This includes support for the `dir` attribute
as well as corresponding CSS properties like `unicode-bidi`. It only
implements right-to-left rendering for inline layout at the moment and
doesn't include support for `dir=auto`. Because of missing features,
this causes quite a few tests to start failing, as references become
incorrect due to right-to-left rendering being active in some cases,
but not others (before it didn't exist at all).

Analysis of most of the new failures:

```
- /css/css-flexbox/gap-001-rtl.html
  /css/css-flexbox/gap-004-rtl.html
 - Require implementing BiDi in Flexbox, because the start and
   end inline margins are opposite the order of items.

- /css/CSS2/bidi-text/direction-applies-to-*.xht
  /css/CSS2/bidi-text/direction-applies-to-002.xht
  /css/CSS2/bidi-text/direction-applies-to-003.xht
  /css/CSS2/bidi-text/direction-applies-to-004.xht
  - Broken due to a bug in tables, not allocating the
    right amount of width for a column.

- /css/css-lists/inline-list.html
  - This fails because we wrongly insert a soft wrap opportunity between the
    start of an inline box and its first content.

- /css/css-text/bidi/bidi-lines-001.html
  /css/css-text/bidi/bidi-lines-002.html
  /css/CSS2/text/bidi-flag-emoji.html
  - We do not fully support unicode-bidi: plaintext

- /css/css-text/text-align/text-align-end-010.html
  /css/css-text/text-align/text-align-justify-006.html
  /css/css-text/text-align/text-align-start-010.html
  /html/dom/elements/global-attributes/*
  - We do not support dir=auto yet.

- /css/css-text/white-space/tab-bidi-001.html
  - Servo doesn't support tab stops

- /css/CSS2/positioning/abspos-block-level-001.html
  /css/css-text/word-break/word-break-normal-ar-000.html
  - Do not yet support RTL layout in block

- /css/css-text/white-space/pre-wrap-018.html
  - Even in RTL contexts, spaces at the end of the line must hang and
    not be reordered

- /css/css-text/white-space/trailing-space-and-text-alignment-rtl-002.html
  - We are letting spaces hang with white-space: pre, but they shouldn't
    hang.
```

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Rakhi Sharma <atbrakhi@igalia.com>

resources/servo.css

@@ -188,6 +188,12 @@ svg > * {
     all: inherit;
 }
 
+*|*::-servo-anonymous-box {
+  unicode-bidi: inherit;
+  direction: inherit;
+  writing-mode: inherit;
+}
+
 /* style for text node. */
 *|*::-servo-legacy-text {
     text-overflow: inherit;

resources/user-agent.css

@@ -130,14 +130,82 @@ wbr { display-outside: break-opportunity; } /* this also has bidi implications *
 nobr wbr { white-space: normal; }
 
 
-[dir]:dir(ltr), bdi:dir(ltr), input[type=tel]:dir(ltr) { direction: ltr; }
-[dir]:dir(rtl), bdi:dir(rtl) { direction: rtl; }
+/* Eventually we will want the following, but currently Servo does not
+   properly parse the :dir pseudo-selector.
+[dir=ltr i], bdi:dir(ltr), input[type=tel]:dir(ltr) { direction: ltr; }
+*/
+[dir=ltr i] { direction: ltr; }
+[dir=rtl i] { direction: rtl; }
+[dir=ltr i], [dir=rtl i], [dir=auto i] { unicode-bidi: isolate; }
 
-address, blockquote, center, div, figure, figcaption, footer, form, header, hr,
-legend, listing, main, p, plaintext, pre, summary, xmp, article, aside, h1, h2,
-h3, h4, h5, h6, hgroup, nav, section, table, caption, colgroup, col, thead,
-tbody, tfoot, tr, td, th, dir, dd, dl, dt, menu, ol, ul, li, bdi, output,
-[dir=ltr i], [dir=rtl i], [dir=auto i] {
+/* To ensure http://www.w3.org/TR/REC-html40/struct/dirlang.html#style-bidi:
+ *
+ * "When a block element that does not have a dir attribute is transformed to
+ * the style of an inline element by a style sheet, the resulting presentation
+ * should be equivalent, in terms of bidirectional formatting, to the
+ * formatting obtained by explicitly adding a dir attribute (assigned the
+ * inherited value) to the transformed element."
+ *
+ * and the rules in http://dev.w3.org/html5/spec/rendering.html#rendering
+ */
+address,
+article,
+aside,
+blockquote,
+body,
+caption,
+center,
+col,
+colgroup,
+dd,
+dir,
+div,
+dl,
+dt,
+fieldset,
+figcaption,
+figure,
+footer,
+form,
+h1,
+h2,
+h3,
+h4,
+h5,
+h6,
+header,
+hgroup,
+hr,
+html,
+legend,
+li,
+listing,
+main,
+marquee,
+menu,
+nav,
+noframes,
+ol,
+p,
+plaintext,
+pre,
+search,
+section,
+summary,
+table,
+tbody,
+td,
+tfoot,
+th,
+thead,
+tr,
+ul,
+xmp
+{
+  unicode-bidi: isolate;
+}
+
+bdi, output {
   unicode-bidi: isolate;
 }