Serde support for re-exported Peniko types

[timtom-dev]

Peniko has a "serde" feature flag that derives serialization for its types. Vello re-exports Peniko, but doesn't have a serde flag. So, if you use the Peniko exported from Vello, there's no way to enable serde for those types.

[DJMcNab]

Vello does not currently directly support serialisation - I think if we were to create a serde flag, it would when we added actual support for doing serialisation properly.

I'd recommend using:

[dependencies]
vello = "..."
peniko = { version = "*", default-features = false, features = ["serde"] }

in the near term, which should enable what you need.
Using a "*" dependency is not normally what you want, but in this case you want the version to be selected by Vello.