parley_engine: Always shape full text, ensuring ShapedText covers the full text (#727)

To shape, you now call `Shaper::shape_text`, which owns the itemization
loop. Together with font selection being infallible (#722), `ShapedText`
is then guaranteed to represent the full source text contiguously.
That's useful to simplify some bookkeeping. See
#715 (comment) for
some prior discussion on this. Because users previously looped items
themselves and would call `Shaper::shape_item`, that meant items could
be skipped or submitted out-of-order.

Because control flow moved into `parley_engine`, some structures are
introduced. `Shaper::shape_text` takes an iterator of items encoding the
span and holding the item's `ShapeOptions` (previously, `parley`
constructed the options inside the loop and passed them as a parameter
to `Shaper::shape_item`). Font selection is now a trait instead of a
callback, with one method for selection and one method called at the
start of each segment.

Note, "segment" is a new name, and encodes maximal spans within the
user's items that have constant bidi level and script. The distinction
between items and segments becomes more important later on, because
browsers reset grapheme segmentation at item boundaries (where shaping
options change), but not at segment boundaries.

Performance on the Japanese benchmarks is improved quite a bit, mostly
because we're smarter about reusing the `FontSelector`. Performance on
Arabic and Latin is about +1.5%, but addressing the TODO I left on the
the item iterator in `parley` appears to make this a net small
performance win on those scripts (i.e., iterating style runs and inline
boxes to figure out where item boundaries are, instead of an `O(chars)`
walk). That's for a separate PR: I just had an LLM address that TODO for
an initial measurement, and have not reviewed that code at all.

<details>
<summary>Benchmarks with the state as in this PR</summary>

```
$ cargo bench --bench main -- compare ../target/benchmarks/main -t 8.

Default Style - arabic 20 characters               [   9.0 us ...   9.2 us ]      +1.42%*
Default Style - latin 20 characters                [   4.5 us ...   4.5 us ]      +1.62%*
Default Style - japanese 20 characters             [   8.6 us ...   8.0 us ]      -7.29%*
Default Style - arabic 1 paragraph                 [  48.6 us ...  49.1 us ]      +1.12%*
Default Style - latin 1 paragraph                  [  17.4 us ...  17.7 us ]      +1.75%*
Default Style - japanese 1 paragraph               [  72.6 us ...  64.9 us ]     -10.66%*
Default Style - arabic 4 paragraph                 [ 202.9 us ... 205.2 us ]      +1.11%*
Default Style - latin 4 paragraph                  [  65.7 us ...  66.3 us ]      +0.82%
Default Style - japanese 4 paragraph               [ 102.4 us ...  90.8 us ]     -11.28%*
Styled - arabic 20 characters                      [  10.1 us ...  10.2 us ]      +1.15%*
Styled - latin 20 characters                       [   5.6 us ...   5.7 us ]      +1.68%*
Styled - japanese 20 characters                    [   9.3 us ...   8.5 us ]      -8.89%*
Styled - arabic 1 paragraph                        [  51.0 us ...  51.7 us ]      +1.25%*
Styled - latin 1 paragraph                         [  21.9 us ...  22.3 us ]      +1.94%*
Styled - japanese 1 paragraph                      [  78.8 us ...  71.1 us ]      -9.77%*
Styled - arabic 4 paragraph                        [ 222.8 us ... 226.6 us ]      +1.72%*
Styled - latin 4 paragraph                         [  84.9 us ...  85.4 us ]      +0.57%
Styled - japanese 4 paragraph                      [ 111.6 us ... 101.0 us ]      -9.55%*
```

</details>

<details>
<summary>Experimental: benchmarks after addressing the item iterator
TODO</summary>

```
$ cargo bench --bench main -- compare ../target/benchmarks/main -t 8.

Default Style - arabic 20 characters               [   9.0 us ...   8.9 us ]      -1.04%*
Default Style - latin 20 characters                [   4.5 us ...   4.5 us ]      -0.29%
Default Style - japanese 20 characters             [   8.7 us ...   7.8 us ]      -9.50%*
Default Style - arabic 1 paragraph                 [  48.6 us ...  48.3 us ]      -0.51%
Default Style - latin 1 paragraph                  [  17.5 us ...  17.5 us ]      +0.09%
Default Style - japanese 1 paragraph               [  72.5 us ...  63.3 us ]     -12.76%*
Default Style - arabic 4 paragraph                 [ 203.0 us ... 201.7 us ]      -0.64%
Default Style - latin 4 paragraph                  [  65.7 us ...  66.2 us ]      +0.64%
Default Style - japanese 4 paragraph               [ 102.5 us ...  88.8 us ]     -13.34%*
Styled - arabic 20 characters                      [  10.1 us ...  10.0 us ]      -0.78%
Styled - latin 20 characters                       [   5.7 us ...   5.6 us ]      -1.37%*
Styled - japanese 20 characters                    [   9.3 us ...   8.4 us ]      -9.73%*
Styled - arabic 1 paragraph                        [  51.3 us ...  50.7 us ]      -1.23%*
Styled - latin 1 paragraph                         [  21.9 us ...  21.9 us ]      +0.03%
Styled - japanese 1 paragraph                      [  78.9 us ...  69.4 us ]     -12.10%*
Styled - arabic 4 paragraph                        [ 222.8 us ... 220.8 us ]      -0.92%
Styled - latin 4 paragraph                         [  84.9 us ...  84.8 us ]      -0.15%
Styled - japanese 4 paragraph                      [ 111.7 us ...  97.8 us ]     -12.45%*
```

</details>

Author: tomcur

parley/src/layout/data.rs

@@ -264,16 +264,7 @@ impl<B: Brush> LayoutData<B> {
     }
 
     /// Push an inline box to the list of items
-    pub(crate) fn push_inline_box(&mut self, index: usize) {
-        // Give the box the same bidi level as the preceding text run
-        // (or else default to 0 if there is not yet a text run)
-        let bidi_level = self
-            .shaped_text
-            .runs()
-            .last()
-            .map(|r| r.bidi_level)
-            .unwrap_or(BidiLevel::new(0));
-
+    pub(crate) fn push_inline_box(&mut self, index: usize, bidi_level: BidiLevel) {
         self.items.push(LayoutItem {
             kind: LayoutItemKind::InlineBox,
             index,