Add support for UTF-8 encoded scenario files

If `encoding = utf8` is specified in system3.ini, system3-sdl2 uses
UTF-8 as its multibyte character encoding. It will be used to run games
translated into non-(Japanese|English) languages.

src/encoding.h

@@ -5,7 +5,7 @@
 
 class Encoding {
  public:
-	static std::unique_ptr<Encoding> create();
+	static std::unique_ptr<Encoding> create(const char* name);
 
 	virtual ~Encoding() = default;