Click entities via space they take up visually

[1j01]

Right now the editor only lets you click on things via their skeletal structure.
It would be more natural to be able to click and drag terrain entities via the inside of the polygon, not just the edge, and more generally, click entities via their visuals.
There's nuance here, like you wouldn't want to drag terrain via its (animated, waving) grass. Entities define their own drawing. How should they define their hit regions for the editor?

Polygons could be special cased for starters. (They're already given special tools. Well, one, the paint tool. The rest work of the tools generalized to arbitrary structures.)
Might want to add a way to force marquee selection when hovering an entity, by the way, as this change would make it harder to use marquee selection where terrain is involved.