[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, 10 insertions, 0 deletions

diff --git a/src/mbgl/style/layer_impl.cpp b/src/mbgl/style/layer_impl.cpp
index 74cc80d253..b345297027 100644
--- a/src/mbgl/style/layer_impl.cpp
+++ b/src/mbgl/style/layer_impl.cpp
@@ -3,6 +3,16 @@
 namespace mbgl {
 namespace style {
 
+std::unique_ptr<Layer> Layer::Impl::copy(const std::string& id_,
+                                         const std::string& ref_,
+                                         const std::string& source_) const {
+    std::unique_ptr<Layer> result = clone();
+    result->baseImpl->id = id_;
+    result->baseImpl->ref = ref_;
+    result->baseImpl->source = source_;
+    return result;
+}
+
 const std::string& Layer::Impl::bucketName() const {
     return ref.empty() ? id : ref;
 }