avcodec/ass,webvttdec: fix handling of backslashes

Backslashes cannot be escaped by a backslash in any ASS renderer,
but unless followed by specific characters it is just printed out.
Insert a word-joiner character after a backslash to break up
active sequences without changing the visual output.

libavcodec/ass.c

@@ -183,9 +183,16 @@ void ff_ass_bprint_text_event(AVBPrint *buf, const char *p, int size,
 
         /* standard ASS escaping so random characters don't get mis-interpreted
          * as ASS */
-        } else if (!keep_ass_markup && strchr("{}\\", *p)) {
+        } else if (!keep_ass_markup && strchr("{}", *p)) {
             av_bprintf(buf, "\\%c", *p);
 
+        /* append word-joiner U+2060 as UTF-8 to break up sequences like \N */
+        } else if (!keep_ass_markup && *p == '\\') {
+            if (p_end - p <= 3 || strncmp(p + 1, "\xe2\x81\xa0", 3))
+                av_bprintf(buf, "\\\xe2\x81\xa0");
+            else
+                av_bprintf(buf, "\\");
+
         /* some packets might end abruptly (no \0 at the end, like for example
          * in some cases of demuxing from a classic video container), some
          * might be terminated with \n or \r\n which we have to remove (for

libavcodec/webvttdec.c

@@ -37,7 +37,7 @@ static const struct {
     {"<i>", "{\\i1}"}, {"</i>", "{\\i0}"},
     {"<b>", "{\\b1}"}, {"</b>", "{\\b0}"},
     {"<u>", "{\\u1}"}, {"</u>", "{\\u0}"},
-    {"{", "\\{"}, {"}", "\\}"}, // escape to avoid ASS markup conflicts
+    {"{", "\\{"}, {"}", "\\}"}, {"\\", "\\\xe2\x81\xa0"}, // escape to avoid ASS markup conflicts
     {"&gt;", ">"}, {"&lt;", "<"},
     {"&lrm;", "\xe2\x80\x8e"}, {"&rlm;", "\xe2\x80\x8f"},
     {"&amp;", "&"}, {"&nbsp;", "\\h"},