Replace C++/CLI components with P/Invoke components (#562)

* Replace C++/CLI components with P/Invoke

* Deleted C++/CLI projects

* Use Utf8StringHandle to marshal UTF-8 strings

* Use plaform-default calling convention

* Handle DLL preloading on Linux and macOS

* Change intermediate and output directories of native projects

* Improve P/Invoke documentation

AssetStudioFBXNative/asfbx_context.h

@@ -0,0 +1,33 @@
+#pragma once
+
+#include <cstdint>
+#include <string>
+#include <unordered_set>
+
+namespace fbxsdk
+{
+	class FbxManager;
+	class FbxScene;
+	class FbxExporter;
+	template<typename T, const int Alignment = 16>
+	class FbxArray;
+	class FbxFileTexture;
+	class FbxSurfacePhong;
+	class FbxPose;
+}
+
+struct AsFbxContext
+{
+
+	fbxsdk::FbxManager* pSdkManager;
+	fbxsdk::FbxScene* pScene;
+	fbxsdk::FbxArray<fbxsdk::FbxFileTexture*>* pTextures;
+	fbxsdk::FbxArray<fbxsdk::FbxSurfacePhong*>* pMaterials;
+	fbxsdk::FbxExporter* pExporter;
+	fbxsdk::FbxPose* pBindPose;
+
+	std::unordered_set<std::string> framePaths;
+
+	AsFbxContext();
+	~AsFbxContext();
+};