fake_gamescope mirrors gamescope 3.16.28's src/pipewire.cpp closely enough to
exercise the requested_size negotiation, MemFd buffers and CORRUPTED frames.
e2e-fake.sh runs it against the client and the mock in the current PipeWire
session; nested-gamescope.sh does the same with a real nested gamescope and
compares the load with the old gstreamer pipeline.
