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.cpp

@@ -22,6 +22,50 @@ private:
 	}
 };
 
+class Utf8Encoding : public Encoding {
+public:
+	int mblen(const unsigned char* s)
+	{
+		if (*s <= 0xbf)
+			return 1;
+		if (*s <= 0xdf)
+			return 2;
+		if (*s <= 0xef)
+			return 3;
+		return 4;
+	}
+
+	int next_codepoint(const unsigned char** str)
+	{
+		int code;
+		const unsigned char *s = *str;
+
+		if (*s <= 0x7f) {
+			code = *s++;
+		} else if (*s <= 0xbf) {
+			// Invalid UTF-8 sequence
+			code = '?';
+			s++;
+		} else if (*s <= 0xdf) {
+			code = (s[0] & 0x1f) << 6 | (s[1] & 0x3f);
+			s += 2;
+		} else if (*s <= 0xef) {
+			code = (s[0] & 0xf) << 12 | (s[1] & 0x3f) << 6 | (s[2] & 0x3f);
+			s += 3;
+		} else if (*s <= 0xf7) {
+			code = (s[0] & 0x7) << 18 | (s[1] & 0x3f) << 12 | (s[2] & 0x3f) << 6 | (s[3] & 0x3f);
+			s += 4;
+		} else {
+			code = 0xfffd;  // REPLACEMENT CHARACTER
+			s++;
+			while (0x80 <= *s && *s <= 0xbf)
+				s++;
+		}
+		*str = s;
+		return code;
+	}
+};
+
 int Encoding::mbslen(const unsigned char* s)
 {
 	int len = 0;
@@ -32,7 +76,18 @@ int Encoding::mbslen(const unsigned char* s)
 	return len;
 }
 
-std::unique_ptr<Encoding> Encoding::create()
+std::unique_ptr<Encoding> Encoding::create(const char* name)
 {
+	if (!strcasecmp(name, "Shift_JIS") ||
+		!strcasecmp(name, "Shift-JIS") ||
+		!strcasecmp(name, "SJIS") ||
+		!strcasecmp(name, "CP932"))
+		return std::make_unique<SjisEncoding>();
+
+	if (!strcasecmp(name, "UTF-8") ||
+		!strcasecmp(name, "UTF8"))
+		return std::make_unique<Utf8Encoding>();
+
+	WARNING("Unrecognized encoding: \"%s\"", name);
 	return std::make_unique<SjisEncoding>();
 }