layout: Shape text only once (#31146)

Shape text during InlineFormattingContext construction rather than doing
it twice during fragment tree construction. This is a step on the way
toward proper font fallback.

This also moves all `TextRun` related code into `text_run.rs` to try to
trim down the size of `inline.rs`.
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- [x] `./mach test-tidy` does not report any errors
- [x] These changes do not require tests because this should only have
performance impacts.

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components/layout_2020/flexbox/construct.rs

@@ -12,7 +12,11 @@ use crate::cell::ArcRefCell;
 use crate::context::LayoutContext;
 use crate::dom::{BoxSlot, LayoutBox, NodeExt};
 use crate::dom_traversal::{Contents, NodeAndStyleInfo, NonReplacedContents, TraversalHandler};
-use crate::formatting_contexts::IndependentFormattingContext;
+use crate::flow::BlockFormattingContext;
+use crate::formatting_contexts::{
+    IndependentFormattingContext, NonReplacedFormattingContext,
+    NonReplacedFormattingContextContents,
+};
 use crate::positioned::AbsolutelyPositionedBox;
 use crate::style_ext::DisplayGeneratingBox;
 
@@ -143,20 +147,29 @@ where
         let mut children = std::mem::take(&mut self.jobs)
             .into_par_iter()
             .map(|job| match job {
-                FlexLevelJob::TextRuns(runs) => ArcRefCell::new(FlexLevelBox::FlexItem(
-                    IndependentFormattingContext::construct_for_text_runs(
-                        &self
-                            .info
-                            .new_replacing_style(anonymous_style.clone().unwrap()),
-                        runs.into_iter().map(|run| crate::flow::inline::TextRun {
-                            base_fragment_info: (&run.info).into(),
-                            text: run.text.into(),
-                            parent_style: run.info.style,
-                            has_uncollapsible_content: false,
-                        }),
+                FlexLevelJob::TextRuns(runs) => ArcRefCell::new(FlexLevelBox::FlexItem({
+                    let runs = runs.into_iter().map(|run| crate::flow::text_run::TextRun {
+                        base_fragment_info: (&run.info).into(),
+                        text: run.text.into(),
+                        parent_style: run.info.style,
+                        has_uncollapsible_content: false,
+                        shaped_text: None,
+                    });
+                    let bfc = BlockFormattingContext::construct_for_text_runs(
+                        runs,
+                        self.context,
                         self.text_decoration_line,
-                    ),
-                )),
+                    );
+                    let info = &self
+                        .info
+                        .new_replacing_style(anonymous_style.clone().unwrap());
+                    IndependentFormattingContext::NonReplaced(NonReplacedFormattingContext {
+                        base_fragment_info: info.into(),
+                        style: info.style.clone(),
+                        content_sizes: None,
+                        contents: NonReplacedFormattingContextContents::Flow(bfc),
+                    })
+                })),
                 FlexLevelJob::Element {
                     info,
                     display,