Add support for filter effects

[nicoburns]

This is based on the implementation from vello_common (https://docs.rs/vello_common/0.0.9/vello_common/filter_effects/index.html). Differences from that implementation:

• Filter and FilterGraph are collapsed into one type (named Filter)
• Filter is not internally Arc'd. The intention is that APIs will explicit accept Arc<Filter>. The advantage of this design is that allows sub-Filters to be composed without allocating intermediate Arcs, and without needing a separate type (FilterGraph).
• FilterFunction and FilterEffect have been removed. You should use what is called FilterPrimitive in vello_common, or convenience constructors on Filter instead. The FilterPrimitive type has been renamed to FilterEffect now that name is available.
• CompoundFilter has been removed.
• All struct-like enums have been split out into standalone structs
• Serde derives have been added to all the types.