Better CesiumImpl implementation.

The previous one wouldn't have called derived-class destructors
properly.

native~/Shared/src/CesiumImpl.h

@@ -1,20 +1,56 @@
 #pragma once
 
-#include <CesiumUtility/ReferenceCounted.h>
+#include <CesiumUtility/Assert.h>
+
+#include <atomic>
+#include <cstdint>
+#include <type_traits>
 
 namespace CesiumForUnityNative {
 
-template <typename TDerived>
-class CesiumImpl
-    : public CesiumUtility::ReferenceCountedThreadSafe<CesiumImpl<TDerived>> {
+template <typename TDerived> class CesiumImpl {
 public:
-  CesiumImpl() = default;
+  /**
+   * @brief Adds a counted reference to this object. Use
+   * {@link CesiumUtility::IntrusivePointer} instead of calling this method
+   * directly.
+   */
+  void addReference() const /*noexcept*/ { ++this->_referenceCount; }
+
+  /**
+   * @brief Removes a counted reference from this object. When the last
+   * reference is removed, this method will delete this instance. Use
+   * {@link CesiumUtility::IntrusivePointer} instead of calling this method
+   * directly.
+   */
+  void releaseReference() const /*noexcept*/ {
+    CESIUM_ASSERT(this->_referenceCount > 0);
+    const int32_t references = --this->_referenceCount;
+    if (references == 0) {
+      delete static_cast<const TDerived*>(this);
+    }
+  }
+
+  /**
+   * @brief Returns the current reference count of this instance.
+   */
+  std::int32_t getReferenceCount() const noexcept {
+    return this->_referenceCount;
+  }
 
   // Prevent copying of impl classes
   CesiumImpl(CesiumImpl&&) = delete;
   CesiumImpl(const CesiumImpl&) = delete;
   CesiumImpl& operator=(CesiumImpl&&) = delete;
   CesiumImpl& operator=(const CesiumImpl&) = delete;
+
+private:
+  CesiumImpl() noexcept = default;
+  ~CesiumImpl() noexcept { CESIUM_ASSERT(this->_referenceCount == 0); }
+
+  friend TDerived;
+
+  mutable std::atomic<std::int32_t> _referenceCount{0};
 };
 
 } // namespace CesiumForUnityNative