Isomp4: support for unseekable downstream

commit d399e25e

qtmux: properly support initial caps nego failure

Scenario:
- gap event causes h264parse to push made up caps that may fail checks
  inside qtmux (e.g missing codec_data).
- the caps event has already been marked as received and is sticky on
  the sink pad
- gst_qt_mux_pad_can_renegotiate() will retrieve the failed caps event
  using gst_pad_get_current_caps() and reject the correct updated caps
  with codec_data.
- Failure!

Keep track of the configured caps ourselves instead of relying on the
sticky event on the pad.

commit bdfb9774

qtmux: support non-seekable downstream mode

Write an mdat per buffer in that case.