Implement parley_core::ShapedText

[tomcur]

This moves shaped run processing from parley to parley_core, and lets parley_core own the shaped results in ShapedText. parley_core now fully owns shaping.

parley stores ShapedText in its LayoutData, dropping its cluster/glyph/etc. vecs. It keeps some run data as a parallel vector to ShapedText::runs, storing some data that parley_core is not aware of.

A large part of the diff (process_clusters) was moved verbatim to core. ShapedContext::push_run is mostly blocks moved out of the old fn push_run that was in parley/src/layout/data.rs.

There's one escape hatch that we should try to get rid of: because parley was mutating shaped data in-place for justification and letter spacing and such, parley_core exposes #[doc(hidden)] mutable access to two ShapedText fields to keep parley happy. This in-place mutation isn't really great in itself, e.g., for justification, parley already has a hack to "unjustify" in-place... but with reshaping I suspect it's untenable.

Benches close to neutral on my machine

Default Style - arabic 20 characters               [   8.8 us ...   9.0 us ]      +2.10%*
Default Style - latin 20 characters                [   4.3 us ...   4.3 us ]      -0.18%
Default Style - japanese 20 characters             [   8.2 us ...   8.3 us ]      +0.88%
Default Style - arabic 1 paragraph                 [  47.7 us ...  47.9 us ]      +0.48%
Default Style - latin 1 paragraph                  [  16.6 us ...  16.6 us ]      +0.40%
Default Style - japanese 1 paragraph               [  70.1 us ...  70.7 us ]      +0.81%
Default Style - arabic 4 paragraph                 [ 200.7 us ... 200.4 us ]      -0.17%
Default Style - latin 4 paragraph                  [  62.2 us ...  62.9 us ]      +1.07%*
Default Style - japanese 4 paragraph               [  99.5 us ... 100.0 us ]      +0.52%
Styled - arabic 20 characters                      [   9.8 us ...  10.0 us ]      +1.85%*
Styled - latin 20 characters                       [   5.4 us ...   5.4 us ]      +0.19%
Styled - japanese 20 characters                    [   8.7 us ...   8.8 us ]      +0.92%
Styled - arabic 1 paragraph                        [  50.2 us ...  50.5 us ]      +0.63%
Styled - latin 1 paragraph                         [  20.9 us ...  21.2 us ]      +1.17%*
Styled - japanese 1 paragraph                      [  75.9 us ...  76.8 us ]      +1.27%*
Styled - arabic 4 paragraph                        [ 219.2 us ... 219.3 us ]      +0.06%
Styled - latin 4 paragraph                         [  81.6 us ...  82.4 us ]      +0.96%
Styled - japanese 4 paragraph                      [ 107.1 us ... 108.4 us ]      +1.23%*

[nicoburns]

There's one escape hatch that we should try to get rid of: because parley was mutating shaped data in-place for justification and letter spacing and such, parley_core exposes #[doc(hidden)] mutable access to two ShapedText fields to keep parley happy.

Why are the fields of ShapedText private at all? Wouldn't it be useful to consumers to have full access to the data?

[tomcur]

Why are the fields of ShapedText private at all? Wouldn't it be useful to consumers to have full access to the data?

The API gives full read access. I'm not sure about mutable access, as there are quite a few invariants that need to be upheld. There's a lot of cross-referencing; e.g., runs contain ranges tiling ShapedText's vecs, and e.g., run advances must be the sum of their glyphs' advances as well as their clusters' advances. Once we have reshaping, there'll be a bit more going on still.

Of course if a user has mutable access and breaks something, that's mostly fine as it's their own fault, but I'm not sure mutation is strongly needed. Parley currently only mutates advances in-place, but it's hacky. E.g. if a layout is justified, the advances are changed, and relayout requires a pass to unjustify first. As the original values are now lost, it's doing the same calculations in reverse; this kind of thing is going to drift because of compounding rounding errors after enough passes.

If ShapedText is a static source of truth for a paragraph, mutations are always applied from a clean slate. With what I have in mind currently, reshaping also only reads ShapedText and doesn't mutate it.

(I think having a bit of a closer look at what exactly the API exposes would be good, after this PR lands we've mostly arrived at the targeted seam between parley_core and parley.)

[tomcur]

This prevents exposing font_types::F2Dot14, but perhaps we should add bytemuck support so users can easily cast slices.

[nicoburns]

I think this type makes sense, but it ought to live in parlance (we have also previously just used plain i16 for this).

[DJMcNab]

The prior art is https://docs.rs/vello_encoding/latest/vello_encoding/type.NormalizedCoord.html.