Use MoveFileExW to implement FioRenameFile on Windows

This is to allow renaming over an existing file

src/fileio.cpp

@@ -383,7 +383,7 @@ void FioCreateDirectory(const std::string &name)
 bool FioRenameFile(const std::string &oldname, const std::string &newname)
 {
 #if defined(_WIN32)
-	return _wrename(OTTD2FS(oldname).c_str(), OTTD2FS(newname).c_str()) == 0;
+	return MoveFileExW(OTTD2FS(oldname).c_str(), OTTD2FS(newname).c_str(), MOVEFILE_COPY_ALLOWED | MOVEFILE_REPLACE_EXISTING) != 0;
 #else
 	return rename(oldname.c_str(), newname.c_str()) == 0;
 #endif

src/sl/saveload.cpp

@@ -2908,11 +2908,6 @@ struct FileWriter : SaveFilter {
 			SlError(STR_GAME_SAVELOAD_ERROR_FILE_NOT_WRITEABLE, stdstr_fmt("Temporary save file does not have expected file size: " PRINTF_SIZE " != " PRINTF_SIZE, (size_t)st.st_size, save_size));
 		}
 
-#if defined(_WIN32)
-		/* Renaming over an existing file is not supported on Windows, manually unlink the target filename first */
-		unlink(this->target_name.c_str());
-#endif
-
 		if (!FioRenameFile(this->temp_name, this->target_name)) SlError(STR_GAME_SAVELOAD_ERROR_FILE_NOT_WRITEABLE, "Failed to rename temporary save file to target name");
 		this->temp_name.clear(); // Now no need to unlink temporary name
 	}