Handle line heights changing within atoms

[tomcur]

LLM Contributions: Investigation, initial implementation, review.

On top of #863. If the line height changes within an atom, we add the spans of all the atom's characters to the line. I believe this matches Blink; but note we do still diverge a bit from Blink regarding the "strut" and accounting styles per-character instead of per-span.

Supercedes #761, #828, #839.

Without this change, the test renders as follows.

Performance

Together with #863, this benches as follows (but there may be some placement noise again).

Line Breaking - arabic 8000 characters, narrow     [ 221.7 us ... 203.0 us ]      -8.44%*
Line Breaking - arabic 8000 characters, wider      [ 121.7 us ... 109.7 us ]      -9.84%*
Line Breaking - arabic 8000 characters, unwrapped  [  92.1 us ...  83.4 us ]      -9.44%*
Line Breaking - latin 8000 characters, narrow      [ 176.7 us ... 168.0 us ]      -4.96%*
Line Breaking - latin 8000 characters, wider       [  86.3 us ...  80.5 us ]      -6.78%*
Line Breaking - latin 8000 characters, unwrapped   [  54.8 us ...  52.8 us ]      -3.77%*
Line Breaking - japanese 8000 characters, narrow   [ 224.7 us ... 213.7 us ]      -4.88%*
Line Breaking - japanese 8000 characters, wider    [ 149.8 us ... 143.3 us ]      -4.35%*
Line Breaking - japanese 8000 characters, unwrapped [ 127.4 us ... 121.2 us ]      -4.89%*
Line Breaking - arabic 8000 characters, wrapped + spacing [ 130.7 us ... 116.8 us ]     -10.67%*
Line Breaking - latin 8000 characters, wrapped + spacing [  94.0 us ...  88.2 us ]      -6.16%*
Line Breaking - japanese 8000 characters, wrapped + spacing [ 155.5 us ... 147.7 us ]      -5.01%*
Page Break - arabic 8000 characters                [  98.2 us ...  88.0 us ]     -10.38%*
Page Break - latin 8000 characters                 [  63.1 us ...  60.2 us ]      -4.58%*
Page Break - japanese 8000 characters              [ 141.0 us ... 131.6 us ]      -6.68%*

Changelog

Fixed

• All line height changes within a run now contribute to the line box.

[nicoburns]

Nit: this could probably skip consecutive chars with the same style?

[tomcur]

It could, but I'm not sure it would save a lot except for very pathological cases. The add_style call already does a scan to check if the style has been added previously. So then, three things have to line up for this to matter: a lot of atoms with more than one character, actually having styles crossing a grapheme or a ligature across styles, and having a lot of styles.

What might be useful is to check per-atom whether it has mixed styles, but the version I tried which did that during line breaking regresses it by 4%.

[tomcur]

I ended up adding a harmless-enough guard that should catch most of those pathological cases, by checking whether the style is different from the atom's first style.

[nicoburns]

Could we document the use of sentinel values here?

[tomcur]

Good point; I've instead simplified it a bit, by checking RunData::has_mixed_style_atoms directly. (There's one remaining use of (usize::MAX, 0) not introduced by this PR, in the Default impl, but I think that one is clear enough as-is.)