movtextenc: fix handling of utf-8 subtitles

See the earlier fix for movtextdec for details. The equivalent bug is
present on the encoder side as well.

We need to track the text length in 'characters' (which seems to really
mean codepoints) to ensure that styles are applied across the correct
ranges.

libavcodec/movtextenc.c

@@ -72,6 +72,7 @@ typedef struct {
     uint8_t style_fontsize;
     uint32_t style_color;
     uint16_t text_pos;
+    uint16_t byte_count;
 } MovTextContext;
 
 typedef struct {
@@ -304,11 +305,34 @@ static void mov_text_color_cb(void *priv, unsigned int color, unsigned int color
      */
 }
 
+static uint16_t utf8_strlen(const char *text, int len)
+{
+    uint16_t i = 0, ret = 0;
+    while (i < len) {
+        char c = text[i];
+        if ((c & 0x80) == 0)
+            i += 1;
+        else if ((c & 0xE0) == 0xC0)
+            i += 2;
+        else if ((c & 0xF0) == 0xE0)
+            i += 3;
+        else if ((c & 0xF8) == 0xF0)
+            i += 4;
+        else
+            return 0;
+        ret++;
+    }
+    return ret;
+}
+
 static void mov_text_text_cb(void *priv, const char *text, int len)
 {
+    uint16_t utf8_len = utf8_strlen(text, len);
     MovTextContext *s = priv;
     av_bprint_append_data(&s->buffer, text, len);
-    s->text_pos += len;
+    // If it's not utf-8, just use the byte length
+    s->text_pos += utf8_len ? utf8_len : len;
+    s->byte_count += len;
 }
 
 static void mov_text_new_line_cb(void *priv, int forced)
@@ -316,6 +340,7 @@ static void mov_text_new_line_cb(void *priv, int forced)
     MovTextContext *s = priv;
     av_bprint_append_data(&s->buffer, "\n", 1);
     s->text_pos += 1;
+    s->byte_count += 1;
 }
 
 static const ASSCodesCallbacks mov_text_callbacks = {
@@ -333,6 +358,7 @@ static int mov_text_encode_frame(AVCodecContext *avctx, unsigned char *buf,
     int i, length;
     size_t j;
 
+    s->byte_count = 0;
     s->text_pos = 0;
     s->count = 0;
     s->box_flags = 0;
@@ -368,7 +394,7 @@ static int mov_text_encode_frame(AVCodecContext *avctx, unsigned char *buf,
         }
     }
 
-    AV_WB16(buf, s->text_pos);
+    AV_WB16(buf, s->byte_count);
     buf += 2;
 
     if (!av_bprint_is_complete(&s->buffer)) {