We now distinguish between input and output audio and image frames. We introduce
`InputAudioRawFrame`, `OutputAudioRawFrame`, `InputImageRawFrame` and
`OutputImageRawFrame` (and other subclasses of those). The input frames usually
come from an input transport and are meant to be processed inside the pipeline
to generate new frames. However, the input frames will not be sent through an
output transport. The output frames can also be processed by any frame processor
in the pipeline and they are allowed to be sent by the output transport.
