Add getter and setter for offset on marker

src/ui/marker.js

@@ -279,6 +279,25 @@ class Marker {
 
         DOM.setTransform(this._element, `translate(-50%, -50%) translate(${this._pos.x}px, ${this._pos.y}px)`);
     }
+
+    /**
+     * Get the marker's offset.
+     * @returns {Point}
+     */
+    getOffset() {
+        return this._offset;
+    }
+
+    /**
+     * Sets the offset of the marker
+     * @param {PointLike} offset The offset in pixels as a {@link PointLike} object to apply relative to the element's center. Negatives indicate left and up.
+     * @returns {Marker} `this`
+     */
+    setOffset(offset: PointLike) {
+        this._offset = Point.convert(offset);
+        this._update();
+        return this;
+    }
 }
 
 module.exports = Marker;

test/unit/ui/marker.test.js

@@ -81,5 +81,16 @@ test('Marker', (t) => {
         t.end();
     });
 
+    t.test('marker\'s offset can be changed', (t) => {
+        const map = createMap();
+        const marker = new Marker(window.document.createElement('div')).setLngLat([-77.01866, 38.888]).addTo(map);
+        const offset = marker.getOffset();
+        t.ok(offset.x === 0 && offset.y === 0, 'default offset');
+        t.ok(marker.setOffset([50, -75]) instanceof Marker, 'marker.setOffset() returns Marker instance');
+        const newOffset = marker.getOffset();
+        t.ok(newOffset.x === 50 &&  newOffset.y === -75, 'marker\'s offset can be updated');
+        t.end();
+    });
+
     t.end();
 });