dectx3gsub: properly increment the line count, each SSA line in MKV (…

…and in libass) requires an unique ReadOrder number. Fix a regression introduced in c72bb7d that was breaking burned subtitles and the creation of incorrect MKV files when the source subtitles were in the tx3g format.

libhb/dectx3gsub.c

@@ -169,7 +169,7 @@ static hb_buffer_t *tx3g_decode_to_ssa(hb_work_private_t *pv, hb_buffer_t *in)
     int charIndex = 0;
     int styleIndex = 0;
 
-    snprintf((char*)dst, maxOutputSize, "%d,,Default,,0,0,0,,", pv->line);
+    snprintf((char*)dst, maxOutputSize, "%d,0,Default,,0,0,0,,", pv->line);
     dst += strlen((char*)dst);
     start = dst;
     for (pos = text, end = text + textLength; pos < end; pos++)
@@ -219,6 +219,8 @@ static hb_buffer_t *tx3g_decode_to_ssa(hb_work_private_t *pv, hb_buffer_t *in)
     *dst = '\0';
     dst++;
 
+    pv->line++;
+
     // Trim output buffer to the actual amount of data written
     out->size = dst - out->data;