Fix EBU STL writing on non-MSVC runtimes

MSVC 2013 and earlier have the _snprintf() function fill fields
completely, eliding the NUL character, if the printed string is
as long as or longer than the given buffer size. This is not the
case in POSIX snprintf(), it always ensures the string is terminated.
This causes several fields in GSI to become truncated and contain
NUL characters which should not appear there.

src/subtitle_format_ebu3264.cpp

@@ -527,8 +527,17 @@ namespace
 	}
 
 #ifdef _MSC_VER
-#define snprintf _snprintf
+#define vsnprintf _vsnprintf
 #endif
+	void fieldprintf(char *field, size_t fieldlen, const char *format, ...)
+	{
+		char buf[16];
+		va_list ap;
+		va_start(ap, format);
+		vsnprintf(buf, fieldlen, format, ap);
+		va_end(ap);
+		memcpy(field, buf, fieldlen);
+	}
 
 	BlockGSI create_header(AssFile const& copy, EbuExportSettings const& export_settings)
 	{
@@ -577,15 +586,15 @@ namespace
 			memcpy(gsi.rd, buf, 6);
 			memcpy(gsi.rn, "00", 2);
 			memcpy(gsi.tng, "001", 3);
-			snprintf(gsi.mnc, 2, "%02u", (unsigned int)export_settings.max_line_length);
+			fieldprintf(gsi.mnc, 2, "%02u", (unsigned int)export_settings.max_line_length);
 			memcpy(gsi.mnr, "99", 2);
 			gsi.tcs = '1';
 			EbuTimecode tcofs = export_settings.timecode_offset;
-			snprintf(gsi.tcp, 8, "%02u%02u%02u%02u", (unsigned int)tcofs.h, (unsigned int)tcofs.m, (unsigned int)tcofs.s, (unsigned int)tcofs.s);
+			fieldprintf(gsi.tcp, 8, "%02u%02u%02u%02u", (unsigned int)tcofs.h, (unsigned int)tcofs.m, (unsigned int)tcofs.s, (unsigned int)tcofs.s);
 		}
 		gsi.tnd = '1';
 		gsi.dsn = '1';
-		memcpy(gsi.co, "NTZ", 3); // neutral zone!
+		memcpy(gsi.co, "XXX", 3);
 		gsi_encoder.Convert(scriptinfo_editing.c_str(), scriptinfo_editing.size(), gsi.en, 32);
 		if (export_settings.text_encoding == EbuExportSettings::utf8)
 			strncpy(gsi.uda, "This file was exported by Aegisub using non-standard UTF-8 encoding for the subtitle blocks. The TTI.TF field contains UTF-8-encoded text interspersed with the standard formatting codes, which are not encoded. GSI.CCT is set to 'U8' to signify this.", sizeof(gsi.uda));
@@ -627,9 +636,9 @@ void Ebu3264SubtitleFormat::WriteFile(const AssFile *src, agi::fs::path const& f
 	BlockGSI gsi = create_header(copy, export_settings);
 
 	BlockTTI &block0 = tti.front();
-	snprintf(gsi.tcf, 8, "%02u%02u%02u%02u", (unsigned int)block0.tci.h, (unsigned int)block0.tci.m, (unsigned int)block0.tci.s, (unsigned int)block0.tci.f);
-	snprintf(gsi.tnb, 5, "%5u", (unsigned int)tti.size());
-	snprintf(gsi.tns, 5, "%5u", (unsigned int)subs_list.size());
+	fieldprintf(gsi.tcf, 8, "%02u%02u%02u%02u", (unsigned int)block0.tci.h, (unsigned int)block0.tci.m, (unsigned int)block0.tci.s, (unsigned int)block0.tci.f);
+	fieldprintf(gsi.tnb, 5, "%5u", (unsigned int)tti.size());
+	fieldprintf(gsi.tns, 5, "%5u", (unsigned int)subs_list.size());
 
 	// write file
 	agi::io::Save f(filename, true);