[core, darwin] Object identity for MGLSource*, MGLStyleLayer*

All `MGLSource` pointers referencing the same logical source will now be object identical; similarly for `MGLStyleLayer`.

include/mbgl/style/layer.hpp

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <mbgl/util/noncopyable.hpp>
+#include <mbgl/util/any.hpp>
 #include <mbgl/style/types.hpp>
 
 #include <memory>
@@ -118,6 +119,11 @@ public:
     // Private implementation
     const std::unique_ptr<Impl> baseImpl;
 
+    // For use in SDK bindings, which store a reference to a platform-native peer
+    // object here, so that separately-obtained references to this object share
+    // identical platform-native peers.
+    any peer;
+
     friend std::string layoutKey(const Layer&);
 };
 

include/mbgl/style/source.hpp

@@ -4,6 +4,7 @@
 #include <mbgl/util/noncopyable.hpp>
 #include <mbgl/util/optional.hpp>
 #include <mbgl/util/range.hpp>
+#include <mbgl/util/any.hpp>
 #include <mbgl/style/types.hpp>
 #include <mbgl/style/query.hpp>
 
@@ -64,6 +65,11 @@ public:
     class Impl;
     const std::unique_ptr<Impl> baseImpl;
 
+    // For use in SDK bindings, which store a reference to a platform-native peer
+    // object here, so that separately-obtained references to this object share
+    // identical platform-native peers.
+    any peer;
+
 protected:
     const SourceType type;
     Source(SourceType, std::unique_ptr<Impl>);

include/mbgl/util/any.hpp

@@ -0,0 +1,10 @@
+#pragma once
+
+#include <linb/any.hpp>
+
+namespace mbgl {
+
+using linb::any;
+using linb::any_cast;
+
+} // namespace mbgl