Added a test fot get extent by tile index

tests/Extent.test.js

@@ -1,4 +1,5 @@
 import { Extent } from "../src/og/Extent";
+import * as mercator from "../src/og/mercator";
 
 test("Testing Extent", () => {
     const extent = new Extent();
@@ -7,4 +8,45 @@ test("Testing Extent", () => {
     Extent.NORTH_POLE_DEG;
     Extent.SOUTH_POLE_DEG;
     expect(extent).toBeTruthy();
+
+});
+
+test("Testing Extent fromTile", () => {
+
+    function getTileX(extent) {
+        return Math.round(
+            Math.abs(-mercator.POLE - extent.southWest.lon) / (extent.northEast.lon - extent.southWest.lon)
+        );
+    }
+
+    function getTileY(extent) {
+        return Math.round(
+            Math.abs(mercator.POLE - extent.northEast.lat) / (extent.northEast.lat - extent.southWest.lat)
+        );
+    }
+
+    function doTest(x, y, z) {
+
+        let srcX = x,
+            srcY = y,
+            srcZ = z;
+
+        let ext = Extent.fromTile(srcX, srcY, srcZ);
+
+        let resX = getTileX(ext),
+            resY = getTileY(ext);
+
+        expect(resX).toStrictEqual(srcX);
+        expect(resY).toStrictEqual(srcY);
+    }
+
+
+    for (let z = 0; z < 5; z++) {
+        let size = Math.pow(2, z);
+        for (let x = 0; x < size; x++) {
+            for (let y = 0; y < size; y++) {
+                doTest(x, y, z);
+            }
+        }
+    }
 });