This change merges `TransformerMuxingVideoRenderer` and
`TransformerTranscodingVideoRenderer` into `TransformerVideoRenderer`.
Besides all features supported by `TransformerTranscodingVideoRenderer`
the new merged `TransformerVideoRenderer` also supports SEF slow motion
flatting without re-encoding like the `TransformerMuxingVideoRenderer`.
To do this, it uses a `SefSlowMotionVideoSampleTransformer` with
the `PassthroughPipeline`.
PiperOrigin-RevId: 410531478
