Add Background layer to map.

This is a limited implementation of layer backgrounds. The API
supports an arbitrary number of arbitrarily named background
layers, but for the time being we only recognise one background
layer named `Background` and of GeoJSON type.

Certain properties, such a colour/color, opacity, etc., are
recognised and applied as feature styles. If not, a default
style is used.

lib/www/client/source/src/views/Map.vue

@@ -267,6 +267,59 @@ const layers = {
         opacity: 0.5
       }
     }
+  }),
+
+  "Background": L.geoJSON(null,{
+    style (feature) {
+      const style = {
+        stroke: undefined,
+        color: "grey",
+        weight: 2,
+        opacity: 0.5,
+        lineCap: undefined,
+        lineJoin: undefined,
+        dashArray: undefined,
+        dashOffset: undefined,
+        fill: undefined,
+        fillColor: "lightgrey",
+        fillOpacity: 0.5,
+        fillRule: undefined
+      };
+
+      for (let key in style) {
+        switch (key) {
+          case "color":
+            style[key] = feature.properties?.colour ?? feature.properties?.color ?? style[key];
+            break;
+          case "fillColor":
+            style[key] = feature.properties?.fillColour ?? feature.properties?.fillColor ?? style[key];
+            break;
+          default:
+            style[key] = feature.properties?.[key] ?? style[key];
+        }
+
+        if (typeof style[key] === "undefined") {
+          delete style[key];
+        }
+      }
+
+      return style;
+
+      return Object.fromEntries(Object.entries(style).map( ([key, defaultValue]) => {
+        function value () {
+          switch (key) {
+            case "color":
+              return feature.properties?.colour ?? feature.properties?.color ?? defaultValue;
+            case "fillColor":
+              return feature.properties?.fillColour ?? feature.properties?.fillColor ?? defaultValue;
+            default:
+              return feature.properties?.[key] ?? defaultValue;
+          }
+        }
+
+        return [key, value()];
+      }));
+    }
   })
 };
 
@@ -372,6 +425,12 @@ export default {
               ? `/project/${this.$route.params.project}/gis/final/line`
               : `/project/${this.$route.params.project}/gis/final/point?${query.toString()}`;
           }
+        },
+        {
+          layer: layers["Background"],
+          url: (query = "") => {
+            return `/project/${this.$route.params.project}/gis/layer/Background`;
+          }
         }
       ],
       labels: {},