layout: Simplify PositioningContext by having it hold a single Vec (#36795)

`PositioningContext` held two vectors, one inside an `Option`, to
differentiate between the version used for a containing block for all
descendants (including `position: absolute` and `position: fixed`) or
only for `position: absolute` descendants. This distinction was really
hard to reason about and required a lot of bookkeeping about what kind
of `PositioningContext` a layout box's parent expected. In addition, it
led to a lot of mistakes.

This change simplifies things so that `PositioningContext` only holds a
single vector. When it comes time to lay out hoisted absolutely
positioned
fragments, the code then:
 - lays out all of them (in the case of a `PositioningContext` for all
   descendants), or
 - only lays out the `position: absolute` descendants and preserves the
   `position: fixed` descendants (in the case the `PositioningContext`
   is only for `position: absolute`.), or
- lays out none of them if the `PositioningContext` was created for
  box that did not establish a containing block for absolutes.

It's possible that this way of dealing with hoisted absolutes is a bit
less efficient, but, the number of these descendants is typically quite
small, so it should not be significant. In addition, this decreases the
size in memory of all `PositioningContexts` which are created in more
situations as time goes on.

Testing: There is a new WPT test with this change.
Fixes: #36696.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/layout/flow/inline/line.rs

@@ -331,7 +331,7 @@ impl LineItemLayout<'_, '_> {
             self.calculate_inline_box_block_start(inline_box_state, space_above_baseline);
 
         let positioning_context_or_start_offset_in_parent =
-            match inline_box.base.new_positioning_context() {
+            match PositioningContext::new_for_layout_box_base(&inline_box.base) {
                 Some(positioning_context) => Either::Left(positioning_context),
                 None => Either::Right(self.current_positioning_context_mut().len()),
             };

components/layout/flow/inline/mod.rs

@@ -2004,9 +2004,7 @@ impl IndependentFormattingContext {
         bidi_level: Level,
     ) {
         // We need to know the inline size of the atomic before deciding whether to do the line break.
-        let mut child_positioning_context = self
-            .new_positioning_context()
-            .unwrap_or_else(|| PositioningContext::new_for_subtree(true));
+        let mut child_positioning_context = PositioningContext::default();
         let IndependentFloatOrAtomicLayoutResult {
             mut fragment,
             baselines,