[core] Introduce Map::getLayer

include/mbgl/map/map.hpp

@@ -149,6 +149,7 @@ public:
 
     AnnotationIDs getPointAnnotationsInBounds(const LatLngBounds&);
 
+    style::Layer* getLayer(const std::string& layerID);
     void addLayer(std::unique_ptr<style::Layer>, const optional<std::string>& beforeLayerID = {});
     void removeLayer(const std::string& layerID);
 

src/mbgl/map/map.cpp

@@ -736,6 +736,10 @@ std::vector<Feature> Map::queryRenderedFeatures(const ScreenBox& box, const opti
 
 #pragma mark - Style API
 
+style::Layer* Map::getLayer(const std::string& layerID) {
+    return impl->style ? impl->style->getLayer(layerID) : nullptr;
+}
+
 void Map::addLayer(std::unique_ptr<Layer> layer, const optional<std::string>& before) {
     impl->view.activate();