[core] Adjust layer source properties to better reflect reality

* Layer source ID is immutable; must be provided to the constructor
* Layer source layer is mutable
* Layers with GeoJSON sources do not have a source layer

While here, make Layer::copy impl-private.

1 files changed, 2 insertions, 3 deletions

diff --git a/include/mbgl/style/layers/symbol_layer.hpp b/include/mbgl/style/layers/symbol_layer.hpp
index 7d98a5ef6b..b21c8deaf7 100644
--- a/include/mbgl/style/layers/symbol_layer.hpp
+++ b/include/mbgl/style/layers/symbol_layer.hpp
@@ -15,14 +15,13 @@ namespace style {
 
 class SymbolLayer : public Layer {
 public:
-    SymbolLayer(const std::string& layerID);
+    SymbolLayer(const std::string& layerID, const std::string& sourceID);
     ~SymbolLayer() final;
 
     // Source
-
-    void setSource(const std::string& sourceID, const std::string& sourceLayer);
     const std::string& getSourceID() const;
     const std::string& getSourceLayer() const;
+    void setSourceLayer(const std::string& sourceLayer);
 
     void setFilter(const Filter&);
     const Filter& getFilter() const;