[default] map snapshotter - add function to get screen coordinate from latlng for snapshot

- Wraps the TransformState for the snapshot so that the snapshotter itself is free to be re-used or destroyed

platform/default/mbgl/map/map_snapshotter.cpp

@@ -3,9 +3,11 @@
 #include <mbgl/actor/actor_ref.hpp>
 #include <mbgl/gl/headless_frontend.hpp>
 #include <mbgl/map/map.hpp>
+#include <mbgl/map/transform_state.hpp>
 #include <mbgl/storage/file_source.hpp>
 #include <mbgl/style/style.hpp>
 #include <mbgl/util/event.hpp>
+#include <mbgl/map/transform.hpp>
 
 namespace mbgl {
 
@@ -62,7 +64,24 @@ MapSnapshotter::Impl::Impl(FileSource& fileSource,
 
 void MapSnapshotter::Impl::snapshot(ActorRef<MapSnapshotter::Callback> callback) {
     map.renderStill([this, callback = std::move(callback)] (std::exception_ptr error) mutable {
-        callback.invoke(&MapSnapshotter::Callback::operator(), error, error ? PremultipliedImage() : frontend.readStillImage());
+
+        // Create lambda that captures the current transform state
+        // and can be used to translate for geographic to screen
+        // coordinates
+        assert (frontend.getTransformState());
+        PointForFn pointForFn { [=, center=map.getLatLng(), transformState = *frontend.getTransformState()] (const LatLng& latLng) {
+            LatLng unwrappedLatLng = latLng.wrapped();
+            unwrappedLatLng.unwrapForShortestPath(center);
+            Transform transform { transformState };
+            return transform.latLngToScreenCoordinate(unwrappedLatLng);
+        }};
+
+        callback.invoke(
+                &MapSnapshotter::Callback::operator(),
+                error,
+                error ? PremultipliedImage() : frontend.readStillImage(),
+                std::move(pointForFn)
+        );
     });
 }
 

platform/default/mbgl/map/map_snapshotter.hpp

@@ -3,6 +3,7 @@
 #include <mbgl/util/image.hpp>
 #include <mbgl/util/thread.hpp>
 #include <mbgl/util/optional.hpp>
+#include <mbgl/util/geo.hpp>
 
 #include <exception>
 #include <memory>
@@ -46,7 +47,8 @@ public:
     void setRegion(const LatLngBounds&);
     LatLngBounds getRegion() const;
 
-    using Callback = std::function<void (std::exception_ptr, PremultipliedImage)>;
+    using PointForFn = std::function<ScreenCoordinate (const LatLng&)>;
+    using Callback = std::function<void (std::exception_ptr, PremultipliedImage, PointForFn)>;
     void snapshot(ActorRef<Callback>);
 
 private: