This also allows us to remove the path generation and interpolation
handling for `rect()` and `xywh()` since that occurs after computation
Regressions in clip-path-interpolation-xywh.html are due to improper
simplification of length-percentage mixes where the length is 0px.
Storing these within `LengthPercentage`, `LengthBox`, and `Variant`
over-complicated things.
We also now use the correct `SerializationMode` when serializing `xywh`
and `rect`
As it turns out, SkPath already behaves the way we need for SVG and HTML
canvas elements. Less work for us, yay! This removes a 5% item from the
profile when scrolling on https://imdb.com/
Note that there's a tiny screenshot test expectation change due to
minor antialiasing differences when we no longer do our redundant
subpath modifications.
Not every user of this requires an `auto` state, but most do.
This has quite a big diff but most of that is mechanical:
LengthPercentageOrAuto has `resolved_or_auto()` instead of `resolved()`,
and `to_px_or_zero()` instead of `to_px()`, to make their output
clearer.
