Implement the xdg-desktop-portal ScreenCast backend via libportal: a
blocking portal handshake (interactive source picker), a PipeWire stream
on the portal's node enumerating BGRx/BGRA/RGBx/RGBA, and a latest-frame
slot handing frames to next_frame(). stop() is thread-safe; teardown
follows the order PipeWire requires. All proxy operations run under the
thread-loop lock to satisfy the protocol extension context checks
('impl_ext_end_proxy called from wrong context' otherwise).
The encoder now accepts padded strides for packed RGB inputs (real
PipeWire row pitches) and maps the new PixelFormat::Bgrx to
AV_PIX_FMT_BGRA.
Add tools/capture_smoke: a manual smoke tool (interactive, not in
meson test) that captures N frames, encodes them, and writes a
self-contained Annex-B elementary stream with prepended SPS/PPS.
Validated manually on Wayland/Hyprland: 2256x1504 H.264 elementary
stream, ffprobe clean. Phase 3 marked complete in docs/PHASES.md.
Allocate packet payloads with av_new_packet so they are freed through the
owning AVBufferRef instead of leaking on every decode. Pad extradata with
AV_INPUT_BUFFER_PADDING_SIZE for FFmpeg's bitstream parsers. Reject
oversized frames before the int cast, handle unexpected EOF in the
EAGAIN-retry loops, and add the missing <limits> include. Document the
valgrind codec check in docs/RUNBOOK.md.
Validated with valgrind: 0 bytes definitely lost, 0 invalid reads.
fegger