[core] Add Snapshotter::cancel method

platform/default/include/mbgl/map/map_snapshotter.hpp

@@ -54,6 +54,7 @@ public:
     using Attributions = std::vector<std::string>;
     using Callback = std::function<void (std::exception_ptr, PremultipliedImage, Attributions, PointForFn, LatLngForFn)>;
     void snapshot(Callback);
+    void cancel();
 
 private:
     class Impl;

platform/default/src/mbgl/map/map_snapshotter.cpp

@@ -232,6 +232,7 @@ public:
     Map& getMap() { return map; }
     const Map& getMap() const { return map; }
     SnapshotterRendererFrontend& getRenderer() { return frontend; }
+    void cancel() { renderStillCallback.reset(); }
 
 private:
     std::unique_ptr<Actor<MapSnapshotter::Callback>> renderStillCallback;
@@ -256,10 +257,6 @@ MapSnapshotter::MapSnapshotter(std::pair<bool, std::string> style,
 
 MapSnapshotter::~MapSnapshotter() = default;
 
-void MapSnapshotter::snapshot(MapSnapshotter::Callback callback) {
-    impl->snapshot(std::move(callback));
-}
-
 void MapSnapshotter::setStyleURL(const std::string& styleURL) {
     impl->getMap().getStyle().loadURL(styleURL);
 }
@@ -310,4 +307,12 @@ const style::Style& MapSnapshotter::getStyle() const {
     return impl->getMap().getStyle();
 }
 
+void MapSnapshotter::snapshot(MapSnapshotter::Callback callback) {
+    impl->snapshot(std::move(callback));
+}
+
+void MapSnapshotter::cancel() {
+    impl->cancel();
+}
+
 } // namespace mbgl