Only decode text direction entities in Sub files

Previously, all entities were decoded in Subtitle files because of a problem with SubtitleEdit and it's /ReverseRtlStartEnd option not being entity-aware.

It actually ends up reversing the `;` of `&rlm;`, instead of the actual value of `&rlm;`. Therefore, I decoded all entities before SubtitleEdit could have processed the Subtitle, but this has caused problems with more advanced formats like TTML and WebVTT as `&lt;` would decode to `<` causing syntax errors, among other problematic characters.

According to the TTML and WebVTT spec, html entity encoding is allowed, and that makes sense or you wouldn't be able to use `<` etc. Any failure for players to show the decoded character would be a player problem and be out of scope with Devine.

devine/core/manifests/hls.py

@@ -316,7 +316,11 @@ class HLS:
                 if isinstance(track, Subtitle):
                     segment_data = try_ensure_utf8(segment_data)
                     if track.codec not in (Subtitle.Codec.fVTT, Subtitle.Codec.fTTML):
-                        segment_data = html.unescape(segment_data.decode("utf8")).encode("utf8")
+                        # decode text direction entities or SubtitleEdit's /ReverseRtlStartEnd won't work
+                        segment_data = segment_data.decode("utf8"). \
+                            replace("‎", html.unescape("‎")). \
+                            replace("‏", html.unescape("‏")). \
+                            encode("utf8")
                 f.write(segment_data)
                 segment_file.unlink()