subassconvert: do not escape likely ASS override tags

Usually SubRip files are not expected to contain ASS override tags,
but unfortunately these files seem to become more common. Example from
a real file:

1
00:00:00,800 --> 00:00:15,000
{\an8}本字幕由 {\c&H26F4FF&}ShinY {\c&HFFAE1A&}深影字幕组{\c&HFFFFFF&} 原创翻译制作

subassconvert.c escaped '{', so that libass displayed the above line
literally.

Try to apply a simple heuristic to detect whether '{' is likely to
start an ASS tag: if the string starts with '{\', and there is a
closing '}', assume it's an ASS tag, otherwise escape the '{' properly.
If it's a likely ASS tag, it's passed through to libass.

The end result is that the above script is displayed in color, while at
the same time legitimate uses of '{' and '}' should work fine. We assume
that nobody uses {...} for commenting text in SubRip files. (This kind
of comment is popular and legal in ASS files, though.)

sub/subassconvert.c

@@ -55,6 +55,11 @@ static void append_text(struct line *dst, char *fmt, ...)
     va_end(va);
 }
 
+static void append_text_n(struct line *dst, char *start, size_t length)
+{
+    append_text(dst, "%.*s", length, start);
+}
+
 static int indexof(const char *s, int c)
 {
     char *f = strchr(s, c);
@@ -88,7 +93,7 @@ static const struct tag_conv {
     {"<b>", "{\\b1}"}, {"</b>", "{\\b0}"},
     {"<u>", "{\\u1}"}, {"</u>", "{\\u0}"},
     {"<s>", "{\\s1}"}, {"</s>", "{\\s0}"},
-    {"{", "\\{"}, {"}", "\\}"},
+    {"}", "\\}"},
     {"\r\n", "\\N"}, {"\n", "\\N"}, {"\r", "\\N"},
 };
 
@@ -417,6 +422,19 @@ void subassconvert_subrip(const char *orig, char *dest, int dest_buffer_size)
                 sp++;
                 line++;
             }
+        } else if (*line == '{') {
+            char *end = strchr(line, '}');
+            if (line[1] == '\\' && end) {
+                // Likely ASS tag, pass them through
+                // Note that ASS tags like {something\an8} are legal too (i.e.
+                // the first character after '{' doesn't have to be '\'), but
+                // consider these fringe cases not worth supporting.
+                append_text_n(&new_line, line, end - line + 1);
+                line = end + 1;
+            } else {
+                append_text(&new_line, "\\{");
+                line++;
+            }
         }
 
         /* Tag conversion code didn't match */