Bring more of the Velato runtime, including Composition, Renderer.

[waywardmonkeys]

This code will need a lot of enhancements and improvements, but it is still very close to what was in Velato with only minimal changes.

This brings an optional dependency on Vello so that the Renderer can be used to create a Vello Scene from the animation and composition data.

[waywardmonkeys]

There's a reasonable argument that some of this should be elsewhere and not in interpoli itself.

But we don't have another place yet, we don't have a clear idea of which pieces would belong where, and we're not sure which pieces will survive the coming refactors and changes ...

So this gets stuff moved to where we can start iterating, and we can always think about splitting up the crate further in the future.

[xorgy]

There's a reasonable argument that some of this should be elsewhere and not in interpoli itself. ...

I've filed #16 to track the work of making interpoli multi-crate.