movtextdec: fix handling of UTF-8 subtitles

Subtitles which contained styled UTF-8 subtitles (i.e. not just 7 bit
ASCII characters) were not handled correctly. The spec mandates that
styling start/end ranges are in "characters". It's not quite clear what
a "character" is supposed to be, but maybe they mean unicode codepoints.

FFmpeg's decoder treated the style ranges as byte idexes, which could
lead to UTF-8 sequences being broken, and the common code dropping the
whole subtitle line.

Change this and count the codepoint instead. This also means that even
if this is somehow wrong, the decoder won't break UTF-8 sequences
anymore. The sample which led me to investigate this now appears to work
correctly.

libavcodec/movtextdec.c

@@ -326,9 +326,24 @@ static const Box box_types[] = {
 
 const static size_t box_count = FF_ARRAY_ELEMS(box_types);
 
+// Return byte length of the UTF-8 sequence starting at text[0]. 0 on error.
+static int get_utf8_length_at(const char *text, const char *text_end)
+{
+    const char *start = text;
+    int err = 0;
+    uint32_t c;
+    GET_UTF8(c, text < text_end ? (uint8_t)*text++ : (err = 1, 0), goto error;);
+    if (err)
+        goto error;
+    return text - start;
+error:
+    return 0;
+}
+
 static int text_to_ass(AVBPrint *buf, const char *text, const char *text_end,
-                        MovTextContext *m)
+                       AVCodecContext *avctx)
 {
+    MovTextContext *m = avctx->priv_data;
     int i = 0;
     int j = 0;
     int text_pos = 0;
@@ -342,6 +357,8 @@ static int text_to_ass(AVBPrint *buf, const char *text, const char *text_end,
     }
 
     while (text < text_end) {
+        int len;
+
         if (m->box_flags & STYL_BOX) {
             for (i = 0; i < m->style_entries; i++) {
                 if (m->s[i]->style_flag && text_pos == m->s[i]->style_end) {
@@ -388,6 +405,12 @@ static int text_to_ass(AVBPrint *buf, const char *text, const char *text_end,
             }
         }
 
+        len = get_utf8_length_at(text, text_end);
+        if (len < 1) {
+            av_log(avctx, AV_LOG_ERROR, "invalid UTF-8 byte in subtitle\n");
+            len = 1;
+        }
+        for (i = 0; i < len; i++) {
             switch (*text) {
             case '\r':
                 break;
@@ -399,6 +422,7 @@ static int text_to_ass(AVBPrint *buf, const char *text, const char *text_end,
                 break;
             }
             text++;
+        }
         text_pos++;
     }
 
@@ -507,10 +531,10 @@ static int mov_text_decode_frame(AVCodecContext *avctx,
             }
             m->tracksize = m->tracksize + tsmb_size;
         }
-        text_to_ass(&buf, ptr, end, m);
+        text_to_ass(&buf, ptr, end, avctx);
         mov_text_cleanup(m);
     } else
-        text_to_ass(&buf, ptr, end, m);
+        text_to_ass(&buf, ptr, end, avctx);
 
     ret = ff_ass_add_rect(sub, buf.str, m->readorder++, 0, NULL, NULL);
     av_bprint_finalize(&buf, NULL);