Render a PNG stream: PNG files back to back, as written by
aether-renderer-capture --stdout or ffmpeg -f image2pipe -c:v png -.
No temp files: the bytes go straight to ffmpeg’s stdin. args.input is ignored.
Like render, but calls on_progress while ffmpeg works: once with frame 0
when encoding starts, then about twice a second, and with 100% at the end.
Use it to drive a progress bar in a GUI or another tool.