common: Add plugin compatibility file

Adds a helper function for converting UTF-8 to UTF-16 on Windows. Also
adds a fopen wrapper function for pathnames that may contain characters
that are not present in the current Windows codepage.

common/include/PluginCompatibility.h

@@ -0,0 +1,42 @@
+/*  PCSX2 - PS2 Emulator for PCs
+ *  Copyright (C) 2018 PCSX2 Dev Team
+ *
+ *  PCSX2 is free software: you can redistribute it and/or modify it under the terms
+ *  of the GNU Lesser General Public License as published by the Free Software Found-
+ *  ation, either version 3 of the License, or (at your option) any later version.
+ *
+ *  PCSX2 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 PCSX2.
+ *  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifdef _WIN32
+#include <windows.h>
+#endif
+
+#include <string>
+#include <vector>
+#include <cstdio>
+
+#ifdef _WIN32
+inline std::wstring convert_utf8_to_utf16(const std::string &utf8_string)
+{
+    int size = MultiByteToWideChar(CP_UTF8, 0, utf8_string.c_str(), -1, nullptr, 0);
+    std::vector<wchar_t> converted_string(size);
+    MultiByteToWideChar(CP_UTF8, 0, utf8_string.c_str(), -1, converted_string.data(), converted_string.size());
+    return {converted_string.data()};
+}
+#endif
+
+// _wfopen has to be used on Windows for pathnames containing non-ASCII characters.
+inline FILE *px_fopen(const std::string &filename, const std::string &mode)
+{
+#ifdef _WIN32
+    return _wfopen(convert_utf8_to_utf16(filename).c_str(), convert_utf8_to_utf16(mode).c_str());
+#else
+    return fopen(filename.c_str(), mode.c_str());
+#endif
+}