Add windowed renderer

app/src/tiles/renderers/windowedRenderer.ts

@@ -0,0 +1,34 @@
+import { Image } from "mapnik";
+
+import { BoundingBox, TileParams, TileRenderer } from "../types";
+import { getXYZ } from "../util";
+
+class WindowedRenderer implements TileRenderer {
+    constructor(
+        /** Bounding box defining the renderer window */
+        public bbox: BoundingBox,
+        
+        /** Renderer to use for tile requests inside window */
+        public insideWindowRenderer: TileRenderer,
+
+        /** Renderer to use for tile requests outside window */
+        public outsideWindowRenderer: TileRenderer
+    ) {}
+
+    getTile(tileParams: TileParams, dataParams: any): Promise<Image> {
+        if(this.isOutsideExtent(tileParams)) {
+            return this.outsideWindowRenderer.getTile(tileParams, dataParams);
+        } else {
+            return this.insideWindowRenderer.getTile(tileParams, dataParams);
+        }
+    }
+
+    private isOutsideExtent({x, y, z}: TileParams) {
+        const xy = getXYZ(this.bbox, z);
+        return xy.minY > y || xy.maxY < y || xy.minX > x || xy.maxX < x;
+    }
+}
+
+export {
+    WindowedRenderer
+};