Get rid of GeomBatch::unioned_polygon. All of the callers happened to be

SVG icons that're circles, so just make a way to turn Bounds into a
Circle. #951

apps/game/src/devtools/story.rs

@@ -103,7 +103,7 @@ impl StoryMapEditor {
                     .scale(0.5)
                     .centered_on(label_center),
             );
-            let hitbox = draw_normal.unioned_polygon();
+            let hitbox = draw_normal.get_bounds().to_circle().to_polygon();
             draw_hovered.append(
                 Text::from(&marker.label)
                     .bg(Color::CYAN)

geom/src/bounds.rs

@@ -2,7 +2,7 @@ use serde::{Deserialize, Serialize};
 
 use aabb_quadtree::geom::{Point, Rect};
 
-use crate::{Distance, LonLat, Polygon, Pt2D, Ring};
+use crate::{Circle, Distance, LonLat, Polygon, Pt2D, Ring};
 
 /// Represents a rectangular boundary of `Pt2D` points.
 #[derive(Clone, Copy, Debug, PartialEq, Serialize, Deserialize)]
@@ -104,6 +104,12 @@ impl Bounds {
         .into_polygon()
     }
 
+    /// Creates a circle centered in the middle of this boundary. Always uses the half width as a
+    /// radius, so if the width and height don't match, this is pretty meaningless.
+    pub fn to_circle(&self) -> Circle {
+        Circle::new(self.center(), Distance::meters(self.width() / 2.0))
+    }
+
     /// The width of this boundary.
     // TODO Really should be Distance
     pub fn width(&self) -> f64 {

map_gui/src/render/turn.rs

@@ -36,7 +36,7 @@ impl DrawMovement {
             let hitbox = if stage.protected_movements.contains(&movement.id) {
                 if movement.id.crosswalk {
                     batch = traffic_signal::walk_icon(movement, prerender);
-                    batch.unioned_polygon()
+                    batch.get_bounds().to_circle().to_polygon()
                 } else {
                     let arrow = movement
                         .geom
@@ -98,7 +98,7 @@ impl DrawMovement {
                     .make_arrow(BIG_ARROW_THICKNESS, ArrowCap::Triangle)
             } else if movement.id.crosswalk {
                 batch = traffic_signal::dont_walk_icon(movement, prerender);
-                batch.unioned_polygon()
+                batch.get_bounds().to_circle().to_polygon()
             } else {
                 // Use circular icons for banned turns
                 let offset = movement

widgetry/src/geom/mod.rs

@@ -123,15 +123,6 @@ impl GeomBatch {
         self
     }
 
-    /// Builds a single polygon covering everything in this batch. Use to create a hitbox.
-    pub fn unioned_polygon(&self) -> Polygon {
-        let mut result = self.list[0].1.clone();
-        for (_, p, _) in &self.list[1..] {
-            result = result.union(p.clone());
-        }
-        result
-    }
-
     /// True when the batch is empty.
     pub fn is_empty(&self) -> bool {
         self.list.is_empty()