[core] Forward ImageManagerObserver::onRemoveUnusedStyleImages to MapObserver interface

include/mbgl/map/map_observer.hpp

@@ -48,6 +48,9 @@ public:
     virtual void onSourceChanged(style::Source&) {}
     virtual void onDidBecomeIdle() {}
     virtual void onStyleImageMissing(const std::string&) {}
+    // This method should return true if unused image can be removed,
+    // false otherwise. By default, unused image will be removed.
+    virtual bool onCanRemoveUnusedStyleImage(const std::string&) { return true; }
 };
 
 } // namespace mbgl

include/mbgl/renderer/renderer_observer.hpp

@@ -35,6 +35,7 @@ public:
     // Style is missing an image
     using StyleImageMissingCallback = std::function<void()>;
     virtual void onStyleImageMissing(const std::string&, StyleImageMissingCallback done) { done(); }
+    virtual void onRemoveUnusedStyleImages(const std::vector<std::string>&) {}
 };
 
 } // namespace mbgl

src/mbgl/map/map_impl.cpp

@@ -179,4 +179,12 @@ void Map::Impl::onStyleImageMissing(const std::string& id, std::function<void()>
     onUpdate();
 }
 
+void Map::Impl::onRemoveUnusedStyleImages(const std::vector<std::string>& unusedImageIDs) {
+    for (const auto& unusedImageID : unusedImageIDs) {
+        if (observer.onCanRemoveUnusedStyleImage(unusedImageID)) {
+            style->removeImage(unusedImageID);
+        }
+    }
+}
+
 } // namespace mbgl

src/mbgl/map/map_impl.hpp

@@ -46,6 +46,7 @@ public:
     void onWillStartRenderingMap() final;
     void onDidFinishRenderingMap() final;
     void onStyleImageMissing(const std::string&, std::function<void()>) final;
+    void onRemoveUnusedStyleImages(const std::vector<std::string>&) final;
 
     // Map
     void jumpTo(const CameraOptions&);

src/mbgl/renderer/renderer_impl.cpp

@@ -762,4 +762,8 @@ void Renderer::Impl::onStyleImageMissing(const std::string& id, std::function<vo
     observer->onStyleImageMissing(id, std::move(done));
 }
 
+void Renderer::Impl::onRemoveUnusedStyleImages(const std::vector<std::string>& unusedImageIDs) {
+    observer->onRemoveUnusedStyleImages(unusedImageIDs);
+}
+
 } // namespace mbgl

src/mbgl/renderer/renderer_impl.hpp

@@ -91,6 +91,7 @@ private:
 
     // ImageManagerObserver implementation
     void onStyleImageMissing(const std::string&, std::function<void()>) override;
+    void onRemoveUnusedStyleImages(const std::vector<std::string>&) final;
 
     void updateFadingTiles();