For large images it always fails with: Invalid buffer size, packet
size 65536 < expected frame_size 3000000
I found out that on linux, the pipe buffer size is 65536, so probably
that's why it fails.
Since I can't seem to be able to raise the pipe buffer size, is there
a way to let ffmpeg know that it should wait until all bytes have been
received, or I explicitly cancel it via CTLR+C or something?