Implement the feDropShadow element

Reusing @johnl's work and adding the missing bits to implement feDropShadow:

• Factor out a function to generate the filter primitives for a drop-shadow effect. This was already present for the drop-shadow filter function.
• Refactor extract_filter_from_filter_node() a bit to use a less hairy iterator, now that individual filter effects can return a Vec<ResolvedPrimitive> instead of a single ResolvedPrimitive.
• Add the new element definition.
• Implement the element's set_attributes.