audio/video: Copy more metas by default in the codec base classes
For audio we copy metas that have no tags at all, or that only have the "audio" and/or "audio-channels" tag. Audio codecs don't change the audio aspect of the stream and in almost all cases don't change the number of channels. They might however change the sample rate (e.g. Opus). Subclasses that change the number of channels will have to override ::transform_meta() accordingly.
For video we copy metas that have no tags at all, or that only have the "video" and/or "video-size" and/or "video-orientation" tag. Video codecs don't change the "video" aspect of the stream and in almost all cases don't change the resolution or orientation. Subclasses that rescale or change the orientation will have to override ::transform_meta() accordingly.