fix: measure traffic inside the requested circle and report missing tiles

Traffic figures for a radius area counted roads anywhere in its bounding
box, and a tile that failed to load silently changed them. Roads are now cut
to the circle, and the flow source reports per request whether any tile
failed so the answer can say its figures are partial.

src/layers/traffic/flowSource.js

@@ -25,21 +25,31 @@ export function createFlowTileSource({
     maxTiles: 16,
   });
   let partial = false;
+  /**
+   * Flow segments for bounds, with whether any covering tile failed to load
+   * for this request.
+   */
+  async function fetchFlowDetail(bounds, { signal, zoom = 12 } = {}) {
+    const result = await tiles.fetchBounds(bounds, {
+      zoom,
+      signal,
+      tiles: bounds.coverage?.coarse,
+    });
+    partial = result.partial;
+    const segments = bounds.coverage
+      ? result.tiles.flat()
+      : result.tiles.flat().flatMap((segment) =>
+          clipTileLine(segment.coords, bounds).map((coords) => ({
+            ...segment,
+            coords,
+          })),
+        );
+    return { segments, partial: result.partial === true };
+  }
   return {
-    async fetchFlowForBounds(bounds, { signal, zoom = 12 } = {}) {
-      const result = await tiles.fetchBounds(bounds, {
-        zoom,
-        signal,
-        tiles: bounds.coverage?.coarse,
-      });
-      partial = result.partial;
-      if (bounds.coverage) return result.tiles.flat();
-      return result.tiles.flat().flatMap((segment) =>
-        clipTileLine(segment.coords, bounds).map((coords) => ({
-          ...segment,
-          coords,
-        })),
-      );
+    fetchFlowDetail,
+    async fetchFlowForBounds(bounds, options) {
+      return (await fetchFlowDetail(bounds, options)).segments;
     },
     getFlowSessionStats: () => ({ ...tiles.getStats(), partial }),
     resetFlowTileCache: () => tiles.clear(),