Clicks, the hover pointer and marquees with the selection tools already
leave out the area that raster effects paint beyond an object. New tests
in crates/engine/src/tests/select_effects.rs hold that in place for drop
shadows, Outer Glow and Gaussian Blur on paths, groups, point type,
symbol art and images, for Shift-click, Direct Selection, the hover
pointer, a drag from a selected object's shadow and a marquee. hit_leaf
gets a doc comment, and docs/target-app-parity.md gets a line citing
https://helpx.adobe.com/illustrator/using/selecting-objects.html.
