Fix flyTo when the final zoom value is not the requested one (mapbox#…

…7222) (mapbox#7223)

* Fix flyTo when final zoom is not requested one (mapbox#7222)

This complete the fix for mapbox#6828 when the final zoom value is
different from the zoom requested.

* Fix final center position and add test case.

src/ui/camera.js

@@ -832,7 +832,7 @@ class Camera extends Evented {
                 tr.pitch = interpolate(startPitch, pitch, k);
             }
 
-            const newCenter = tr.unproject(from.add(delta.mult(u(s))).mult(scale));
+            const newCenter = k === 1 ? center : tr.unproject(from.add(delta.mult(u(s))).mult(scale));
             tr.setLocationAtPoint(tr.renderWorldCopies ? newCenter.wrap() : newCenter, pointAtOffset);
 
             this._fireMoveEvents(eventData);

test/unit/ui/camera.test.js

@@ -912,6 +912,24 @@ test('camera', (t) => {
             t.end();
         });
 
+        t.test('Zoom out from the same position to the same position with animation', (t) => {
+            const pos = { lng: 0, lat: 0 };
+            const camera = createCamera({zoom: 20, center: pos});
+            const stub = t.stub(browser, 'now');
+
+            camera.once('zoomend', () => {
+                t.deepEqual(fixedLngLat(camera.getCenter()), fixedLngLat(pos));
+                t.equal(camera.getZoom(), 19);
+                t.end();
+            });
+
+            stub.callsFake(() => 0);
+            camera.flyTo({ zoom: 19, center: pos, duration: 2 });
+
+            stub.callsFake(() => 3);
+            camera.simulateFrame();
+        });
+
         t.test('rotates to specified bearing', (t) => {
             const camera = createCamera();
             camera.flyTo({ bearing: 90, animate: false });