Knows which subtitle lines are being spoken in this scene right now. Sounds
carry their own subtitle tracks (Sandbox.SoundFile.Subtitles, authored in
the sound editor) - Sandbox.SubtitlesGameObjectSystem.GetActive(System.Collections.Generic.List{Sandbox.SubtitlesGameObjectSystem.Line}) samples the ones playing in this
scene. The built-in overlay renders them word by word; to draw your own UI
instead, turn off "Show UI" in the project's platform settings and build it
on this system.