Maps · Layers: routes, GeoJSON, heat and clusters

MapView takes layers over its tiles and under its pins. RouteLine runs
along places the short way across the date line. read_geojson reads RFC
7946 to features, strictly: a position past the globe, a line of one
position or a ring that does not close fails with its reason, and a
feature placed nowhere keeps no geometry. GeoJsonLayer washes areas as
one path per polygon, so holes stay open, outlines them, strokes lines
and dots points. GeoHeatmap spreads weighted places in a quartic kernel
over small cells, scaled to the densest spot in view. Lines and areas
unwrap across the date line and repeat on every world in view.
cluster_markers gathers pins by squares of the world, so a pan keeps
each group; a cluster shows its count, and its press shows its members
whole, never farther out. A zoom that may regroup them hands a focused
pin's or cluster's focus to the map.

src/theme/maps.rs

@@ -11,6 +11,20 @@ pub struct MapSizes {
     pub popup: Rems,
     /// Room a popup keeps from the map's edges and from its point.
     pub margin: Rems,
+    /// A route's stroke.
+    pub route: Rems,
+    /// A GeoJSON line's and outline's stroke.
+    pub line: Rems,
+    /// A GeoJSON point's side.
+    pub dot: Rems,
+    /// A heat cell's side.
+    pub heat_cell: Rems,
+    /// How far a heat point spreads.
+    pub heat_reach: Rems,
+    /// A cluster's side.
+    pub cluster: Rems,
+    /// The square of the world that gathers pins into one cluster.
+    pub gather: Rems,
 }
 
 impl Theme {
@@ -19,6 +33,13 @@ impl Theme {
             tile: px(256.0),
             popup: px_to_rems(240.0),
             margin: px_to_rems(8.0),
+            route: px_to_rems(3.0),
+            line: px_to_rems(1.5),
+            dot: px_to_rems(8.0),
+            heat_cell: px_to_rems(4.0),
+            heat_reach: px_to_rems(28.0),
+            cluster: px_to_rems(32.0),
+            gather: px_to_rems(56.0),
         }
     }
 }