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

AssetStudioFBXWrapper/Fbx.cs

@@ -0,0 +1,58 @@
+using AssetStudio.FbxInterop;
+using AssetStudio.PInvoke;
+using System.IO;
+
+namespace AssetStudio
+{
+    public static partial class Fbx
+    {
+
+        static Fbx()
+        {
+            DllLoader.PreloadDll(FbxDll.DllName);
+        }
+
+        public static Vector3 QuaternionToEuler(Quaternion q)
+        {
+            AsUtilQuaternionToEuler(q.X, q.Y, q.Z, q.W, out var x, out var y, out var z);
+            return new Vector3(x, y, z);
+        }
+
+        public static Quaternion EulerToQuaternion(Vector3 v)
+        {
+            AsUtilEulerToQuaternion(v.X, v.Y, v.Z, out var x, out var y, out var z, out var w);
+            return new Quaternion(x, y, z, w);
+        }
+
+        public static class Exporter
+        {
+
+            public static void Export(string path, IImported imported, bool eulerFilter, float filterPrecision,
+                bool allNodes, bool skins, bool animation, bool blendShape, bool castToBone, float boneSize, float scaleFactor, int versionIndex, bool isAscii)
+            {
+                var file = new FileInfo(path);
+                var dir = file.Directory;
+
+                if (!dir.Exists)
+                {
+                    dir.Create();
+                }
+
+                var currentDir = Directory.GetCurrentDirectory();
+                Directory.SetCurrentDirectory(dir.FullName);
+
+                var name = Path.GetFileName(path);
+
+                using (var exporter = new FbxExporter(name, imported, allNodes, skins, castToBone, boneSize, scaleFactor, versionIndex, isAscii))
+                {
+                    exporter.Initialize();
+                    exporter.ExportAll(blendShape, animation, eulerFilter, filterPrecision);
+                }
+
+                Directory.SetCurrentDirectory(currentDir);
+            }
+
+        }
+
+    }
+}