Pass encoded frames through in the media player instead of decoding and re-encoding #368

MediaRecorder already writes encoded frames to a file without decoding or re-encoding. Playback does not: MediaPlayer / MediaFileSource decode with FFmpeg and push raw frames into a CustomVideoSource / CustomAudioSource, which WebRTC then encodes again.

When the source codec matches the one the sender negotiated, the player could hand FFmpeg's demuxed packets straight to the encoded frame injectors from #365 instead, keeping decoding as the fallback. For example, an H.264 RTSP camera would be forwarded without a decode and an encode, saving CPU and avoiding generation loss.

Things to handle:

• H.264/H.265 from MP4 need Annex B (h264_mp4toannexb / hevc_mp4toannexb).
• Streams with B-frames cannot pass through, as WebRTC does not reorder frames.
• Key frame requests can only be met at the source's next key frame (seek for files, wait for live streams).
• A fixed source cannot follow the bitrate the sender allows; at most it can drop frames.