Override Subtitle download method to convert fTTML/fVTT to TTML/VTT

We want to force convert these Subtitle formats to their respective normal formats as the way they download are not actually usable as-is. As in, even if the user wanted to keep the original Subtitle format, these formats wouldn't be usable as-is.

devine/core/tracks/subtitle.py

@@ -4,11 +4,13 @@ import re
 import subprocess
 from collections import defaultdict
 from enum import Enum
+from functools import partial
 from io import BytesIO
 from pathlib import Path
-from typing import Any, Iterable, Optional
+from typing import Any, Callable, Iterable, Optional
 
 import pycaption
+import requests
 from construct import Container
 from pycaption import Caption, CaptionList, CaptionNode, WebVTTReader
 from pycaption.geometry import Layout
@@ -144,6 +146,16 @@ class Subtitle(Track):
             track_name += flag
         return track_name or None
 
+    def download(self, session: requests.Session, prepare_drm: Callable, progress: partial) -> None:
+        super().download(session, prepare_drm, progress)
+        if not self.path:
+            return
+
+        if self.codec == Subtitle.Codec.fTTML:
+            self.convert(Subtitle.Codec.TimedTextMarkupLang)
+        elif self.codec == Subtitle.Codec.fVTT:
+            self.convert(Subtitle.Codec.WebVTT)
+
     def convert(self, codec: Subtitle.Codec) -> Path:
         """
         Convert this Subtitle to another Format.