supportsMux method
No first-party web muxer yet: createMuxer returns null, so advertising
mux support would make the negotiator select this backend and then get
null. The real fMP4/WebM web muxer (MediaRecorder / a pure-Dart writer)
lands in a later phase; flip this back on when createMuxer is real.
Implementation
@override
bool supportsMux(Container container) => false;