Resolve and render filters

This finally separates filter primitives into resolve() and render() steps.

First, all FeFoo implement a FilterElement::resolve method, which yields a PrimitiveParams enum. That enum has one variant for each of the resolved primitives. This makes primitives extract all the values they need from their corresponding nodes and their ComputedValues.

Then, at render() time, we don't have nodes or computed values anymore; we operate on final values only...

... except for the Length inside the base Primitive structs, which need to be normalized. One day we may do that in advance, but not just now.