Fix #12648: Ensure all uses of std::filesystem::path use native encoding. (#12650)

std::filesystem::path does not expect UTF-8 strings, so they must be converted to native format first (and back to utf-8 after.)

src/signature.cpp

@@ -245,7 +245,7 @@ static bool _ValidateSignatureFile(const std::string &filename)
 		return false;
 	}
 
-	std::string dirname = std::filesystem::path(filename).parent_path().string();
+	std::string dirname = FS2OTTD(std::filesystem::path(OTTD2FS(filename)).parent_path());
 
 	for (auto &signature : signatures["files"]) {
 		const std::string sig_filename = dirname + PATHSEPCHAR + signature["filename"].get<std::string>();