capture(wayland): fix black screen and pts timing on gst-launch

[domenkozar]

Keep the gst-launch CLI for the Wayland portal path, but fold in two fixes found while prototyping a go-gst version:

• vapostproc ! video/x-raw,format=I420 imports the portal's DMA-BUF via VA-API (plain videoconvert fails to negotiate DMA-BUF on many drivers) and forces 4:2:0. RGB screens otherwise make x264enc emit "High 4:4:4 Predictive", which most receiver decoders reject (black screen).
• videorate drop-only=true skip-to-first=true ! framerate caps re-stamps buffers onto a regular fps timeline; the portal can deliver pts=0, which confuses encoder/muxer timing.

No new dependencies — the build stays pure Go (no CGO); GStreamer is still only invoked as the gst-launch-1.0 external binary.

Verified end-to-end: mirrors a Wayland desktop to a non-Apple AirPlay 2 receiver (Hisense), yuv420p, no crash.

[domenkozar]

@omarroth this is ready for review when you have a moment.

It swaps the gst-launch subprocess for the go-gst library and fixes the black-screen issues on modern Wayland/Mesa (DMA-BUF import via vapostproc, 4:2:0 I420, PTS re-stamp). It also fixes two in-process cgo crashes that the CLI path never hit: a missing gst.Init() (segfault in gst_parse_launch) and an appsink SetCallbacks use-after-free (now a blocking PullSample loop).

Verified end-to-end mirroring a niri desktop to a non-Apple AirPlay 2 receiver (Hisense), yuv420p, no crash. Happy to adjust anything.

[domenkozar]

@omarroth ready for review

[omarroth]

Works for me 👍. Could you also provide the player (device and wayland environment) and model of HiSense device you're attempting to stream to? If possible I'd like to start building a compatibility matrix for preventing breakage as things get moved around.

Also I expect you're aware from #8, but please make sure you're comfortable providing this code under the terms of the LGPL or later since the license has changed.

Otherwise good to merge.

[domenkozar]

Sure! Sender setup for the compatibility matrix:

• Device: laptop with AMD Ryzen 7 7840S (Radeon 780M iGPU, amdgpu kernel driver, Mesa VA-API)
• OS: NixOS 26.05, kernel 7.0.10
• Compositor: niri 26.04 (Wayland)
• Screen capture: xdg-desktop-portal 1.20.4 with the xdg-desktop-portal-gnome backend, PipeWire 1.6.5
• GStreamer: 1.26.11 (the gst-launch-1.0 binary the pipeline shells out to)

Receiver: Hisense 55A6QU. From its AirPlay advertisement: features=0x38bcf46007f8ad0, no FairPlay SAP (FPSAP bit not advertised), so the session falls back to pair-verify DataStream setup.

On the license: yes, I'm comfortable contributing this under LGPL-3.0-or-later.