[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