Correctly open files with non-ASCII paths on Windows

This fixes #976, fixes #974, fixes #444.

On Windows the standard stream open() functions expect bytes encoded in
current system encoding, not UTF8. Since we're dealing with UTF8
throughout the application this results in wrong paths being passed and
failure to open files. As a solution, we convert the paths to UTF16 via
the WCHAR character type and use the special Windows-specific overloads
of open() functions.

src/lib/io/fstream.cpp

@@ -0,0 +1,57 @@
+/*
+    barrier -- mouse and keyboard sharing utility
+    Copyright (C) Barrier contributors
+
+    This package is free software; you can redistribute it and/or
+    modify it under the terms of the GNU General Public License
+    found in the file LICENSE that should have accompanied this file.
+
+    This package 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, see <http://www.gnu.org/licenses/>.
+*/
+
+#include "fstream.h"
+#if SYSAPI_WIN32
+#include "common/win32/encoding_utilities.h"
+#endif
+#include <fstream>
+
+namespace barrier {
+
+namespace {
+
+template<class Stream>
+void open_utf8_path_impl(Stream& stream, const std::string& path, std::ios_base::openmode mode)
+{
+#if SYSAPI_WIN32
+    // on Windows we need to use a private constructor from wchar_t* string.
+    auto wchar_path = utf8_to_win_char(path);
+    stream.open(wchar_path.data(), mode);
+#else
+    stream.open(path.c_str(), mode);
+#endif
+}
+
+} // namespace
+
+void open_utf8_path(std::ifstream& stream, const std::string& path, std::ios_base::openmode mode)
+{
+    open_utf8_path_impl(stream, path, mode);
+}
+
+void open_utf8_path(std::ofstream& stream, const std::string& path, std::ios_base::openmode mode)
+{
+    open_utf8_path_impl(stream, path, mode);
+}
+
+void open_utf8_path(std::fstream& stream, const std::string& path, std::ios_base::openmode mode)
+{
+    open_utf8_path_impl(stream, path, mode);
+}
+
+} // namespace barrier