Fix T93960: Asset Catalogs I/O fails with unicode file paths on Windows

On Windows, encode file paths as UTF-16 before trying to open the file
for reading/writing.

This introduces a new class `blender::fstream`, which wraps
`std::fstream` and provides this UTF-16 encoding. This class should also
be used in other areas, like the Alembic importer/exporter.

Manifest Task: T93960

Reviewed By: JacquesLucke

Differential Revision: https://developer.blender.org/D13633

source/blender/blenlib/BLI_fileops.hh

@@ -0,0 +1,52 @@
+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ */
+
+/** \file
+ * \ingroup bli
+ * \brief File and directory operations.
+ */
+
+#pragma once
+
+#ifndef __cplusplus
+#  error This is a C++ header
+#endif
+
+#include "BLI_fileops.h"
+#include "BLI_string_ref.hh"
+
+#include <fstream>
+#include <string>
+
+namespace blender {
+
+/**
+ * std::fstream subclass that handles UTF-16 encoding on Windows.
+ *
+ * For documentation, see https://en.cppreference.com/w/cpp/io/basic_fstream
+ */
+class fstream : public std::fstream {
+ public:
+  fstream() = default;
+  explicit fstream(const char *filepath,
+                   std::ios_base::openmode mode = ios_base::in | ios_base::out);
+  explicit fstream(const std::string &filepath,
+                   std::ios_base::openmode mode = ios_base::in | ios_base::out);
+
+  void open(StringRefNull filepath, ios_base::openmode mode = ios_base::in | ios_base::out);
+};
+
+}  // namespace blender