torbrowser: support H.264 playback

Fixes https://github.com/NixOS/nixpkgs/issues/20840

Some notes for future reference:
- Firefox only supports legacy gstreamer (0.10)
- gmp and ffmpeg are appearantly used by gst-ffmpeg so must be in the
  library search path
- Setting GST_DEBUG="*:3" or so was useful in figuring out what to add