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>

components/layout_2020/flow/root.rs

@@ -123,7 +123,7 @@ impl BoxTree {
         #[allow(clippy::enum_variant_names)]
         enum UpdatePoint {
             AbsolutelyPositionedBlockLevelBox(ArcRefCell<BlockLevelBox>),
-            AbsolutelyPositionedInlineLevelBox(ArcRefCell<InlineItem>),
+            AbsolutelyPositionedInlineLevelBox(ArcRefCell<InlineItem>, usize),
             AbsolutelyPositionedFlexLevelBox(ArcRefCell<FlexLevelBox>),
         }
 
@@ -183,11 +183,12 @@ impl BoxTree {
                     },
                     LayoutBox::InlineBox(_) => return None,
                     LayoutBox::InlineLevel(inline_level_box) => match &*inline_level_box.borrow() {
-                        InlineItem::OutOfFlowAbsolutelyPositionedBox(_)
+                        InlineItem::OutOfFlowAbsolutelyPositionedBox(_, text_offset_index)
                             if box_style.position.is_absolutely_positioned() =>
                         {
                             UpdatePoint::AbsolutelyPositionedInlineLevelBox(
                                 inline_level_box.clone(),
+                                *text_offset_index,
                             )
                         },
                         _ => return None,
@@ -219,10 +220,14 @@ impl BoxTree {
                                 out_of_flow_absolutely_positioned_box,
                             );
                     },
-                    UpdatePoint::AbsolutelyPositionedInlineLevelBox(inline_level_box) => {
+                    UpdatePoint::AbsolutelyPositionedInlineLevelBox(
+                        inline_level_box,
+                        text_offset_index,
+                    ) => {
                         *inline_level_box.borrow_mut() =
                             InlineItem::OutOfFlowAbsolutelyPositionedBox(
                                 out_of_flow_absolutely_positioned_box,
+                                text_offset_index,
                             );
                     },
                     UpdatePoint::AbsolutelyPositionedFlexLevelBox(flex_level_box) => {