Implement the blur() filter function

This finally gets us to the point where we can implement CSS filter functions easily.

Based on work from !387 (closed), I've finished refactoring the filters code like this:

• At rendering time, an element's filters get resolved into a FilterSpec. This contains all the parameters for CSS filtering - bounds, units, and the list of filter primitives.
• While rendering each filter primitive, the code does not have to pull out parameters from nodes/values/etc. - they are all part of the PrimitiveParams variants, and generally in the form of fundamental types (e.g. an already-normalized f64 instead of Length).

The code has all the machinery necessary to transform an attribute like filter="blur(5)" into a FilterSpec with a PrimitiveParams::GaussianBlur inside it.

There's a new source file, src/filter_func.rs, where we should put all the other filter functions:

• FilterFunc is an enum that just has a Blur variant right now; it should have one for each of the [functions in the spec]((https://www.w3.org/TR/filter-effects/#filter-functions).
• FilterFunc::parse needs to be expanded for each additional function.
• FilterFunc::to_filter_spec does the conversion from high-level functions to low-level FilterSpec and its primitives. For example, the drop_shadow function will expand to a blur/offset/merge/etc.