ExportMem::copy_framebuffer may not capture composited client surfaces.
Read pixels directly from the pixman Image's CPU memory after rendering,
since PixmanRenderer composites in-place. This should fix missing
client windows in the remote display.
Start QUIC server in wrsrvd main.rs before dispatching to backend,
passing NetworkHandle to the headless backend. The headless render loop
now encodes each frame as XOR diff against the previous frame, compresses
damage regions with zstd, and sends FrameUpdate messages to connected
clients via the network channel.
Network input from remote clients is drained each iteration of the main
event loop and injected into the compositor via inject_network_input().
Connection state (client connected/disconnected) is tracked to avoid
encoding frames when no client is listening.
Restructure wrsrvd to support two backends: a headless PixmanRenderer
(default) for running without a display server, and the existing Winit
backend (via --backend winit). The render logic is split into per-backend
modules, and the old render.rs is removed.
