Add run-time plug-in layer support to iOS (#3430)

include/mbgl/layermanager/layer_manager.hpp

@@ -67,6 +67,15 @@ public:
      */
     static const bool annotationsEnabled;
 
+    /**
+     * Enables a layer type for JSON style only.
+     *
+     * We might not want to expose runtime API for some layer types
+     * in order to save binary size (the corresponding SDK layer wrappers
+     * should be excluded from the project build).
+     */
+    virtual void addLayerTypeCoreOnly(std::unique_ptr<mbgl::LayerFactory>);
+
 protected:
     virtual ~LayerManager() = default;
     virtual LayerFactory* getFactory(const std::string& type) noexcept = 0;

include/mbgl/style/layer.hpp

@@ -90,6 +90,10 @@ struct LayerTypeInfo {
     } tileKind;
 };
 
+// Added this to support plugins and that their LayerTypeInfo isn't the same point
+// across the board
+bool layerTypeInfoEquals(const mbgl::style::LayerTypeInfo* one, const mbgl::style::LayerTypeInfo* other);
+
 /**
  * The runtime representation of a [layer](https://maplibre.org/maplibre-style-spec/#layers)
  * from the MapLibre Style Spec.