Extract embedded fonts if available

Set ass_set_extract_fonts(ass_library, 1); the same way as tests/tools use to load embedded fonts in .ass
Execute ass_clear_fonts(ass_library); on removeTrack() to clear any in-memory fonts that have been loaded.

Author: WeebDataHoarder

src/SubtitleOctopus.cpp

@@ -295,6 +295,7 @@ class SubtitleOctopus {
         }
 
         ass_set_message_cb(ass_library, msg_callback, NULL);
+        ass_set_extract_fonts(ass_library, 1);
 
         ass_renderer = ass_renderer_init(ass_library);
         if (!ass_renderer) {
@@ -334,6 +335,7 @@ class SubtitleOctopus {
             ass_free_track(track);
             track = NULL;
         }
+        ass_clear_fonts(ass_library);
     }
     /* TRACK */