[librptext] conversion_win32.cpp, cpN_to_utf8(): Call cpN_to_utf16().

Windows uses UTF-16 natively, so the UTF-8 conversion function was
essentially cpN_to_utf16() followed by "convert to UTF-8".

Rewrite the function to actually call cpN_to_utf16() so we can
de-duplicate a fairly big chunk of code.

src/librptext/conversion_win32.cpp

@@ -101,60 +101,8 @@ string cpN_to_utf8(unsigned int cp, const char *str, int len, unsigned int flags
 		return cpRP_to_utf8(cp, str, len);
 	}
 
-	len = check_NULL_terminator(str, len);
-	DWORD dwFlags = 0;
-	if (flags & TEXTCONV_FLAG_CP1252_FALLBACK) {
-		// Fallback is enabled.
-		// Fail on invalid characters in the first pass.
-		dwFlags = MB_ERR_INVALID_CHARS;
-	}
-
 	// Convert from `cp` to UTF-16.
-	u16string s_wcs;
-
-	if ((flags & TEXTCONV_FLAG_JIS_X_0208) && len >= 1) {
-		// Check if the string might be JIS X 0208.
-		// If it is, make it EUC-JP compatible, then convert it.
-		bool is0208 = false;
-		// Heuristic: First character should be 0x21-0x24.
-		if (*str >= 0x21 && *str <= 0x24) {
-			is0208 = true;
-			const char *const p_end = str + len;
-			for (const char *p = str + 1; p < p_end; p++) {
-				const uint8_t chr = static_cast<uint8_t>(*p);
-				if (chr == 0) {
-					// End of string
-					break;
-				} else if (chr & 0x80) {
-					// High bit cannot be set
-					is0208 = false;
-				}
-			}
-		}
-
-		if (is0208) {
-			// Make the string EUC-JP compatible.
-			string eucJP(str, 0, len);
-			for (char &c : eucJP) {
-				c |= 0x80;
-			}
-			if (W32U_mbs_to_UTF16(s_wcs, eucJP.c_str(), eucJP.size(), 20932, dwFlags) != 0) {
-				s_wcs.clear();
-			}
-		}
-	}
-
-	if (s_wcs.empty()) {
-		if (W32U_mbs_to_UTF16(s_wcs, str, len, cp, dwFlags) != 0) {
-			if (flags & TEXTCONV_FLAG_CP1252_FALLBACK) {
-				// Try again using cp1252.
-				if (W32U_mbs_to_UTF16(s_wcs, str, len, 1252, 0) != 0) {
-					// Failed.
-					s_wcs.clear();
-				}
-			}
-		}
-	}
+	u16string s_wcs = cpN_to_utf16(cp, str, len, flags);
 
 	string s_mbs;
 	if (!s_wcs.empty()) {