scrap/wayland: insert videoconvert to fix screencast on COSMIC / DMA-BUF portals (#15063)

On Wayland compositors whose xdg-desktop-portal backend exposes screencast
frames as DMA-BUF buffers — notably xdg-desktop-portal-cosmic 0.1.0 on
Pop!_OS 24.04 / COSMIC — inbound screen capture fails. PipeWireRecorder
links pipewiresrc directly to an appsink whose caps only accept
video/x-raw BGRx/RGBx in system memory. That format set is too narrow for
the portal's buffer-type / modifier negotiation, which collapses with:

  pw.link: negotiating -> error no more output formats (-22)
  gstpipewiresrc: stream error: no more output formats
  gstbasesrc: streaming stopped, reason not-negotiated (-4)
  ERROR src/server/wayland.rs: Failed scrap Element failed to change its state

Inserting a videoconvert element between pipewiresrc and appsink widens
the negotiable format set to any system-memory video/x-raw format, giving
the portal room to settle on a format it can deliver via its SHM path.
videoconvert then converts to the BGRx/RGBx the appsink expects.

Verified on Pop!_OS 24.04 / COSMIC with gst-launch, before and after:

  # fails (current behaviour):
  gst-launch-1.0 pipewiresrc path=N ! video/x-raw,format=BGRx ! fakesink
  # works (with this change):
  gst-launch-1.0 pipewiresrc path=N ! videoconvert ! video/x-raw,format=BGRx ! fakesink

After the change, inbound connections capture and stream the desktop
normally and the "Failed scrap" error no longer occurs.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

libs/scrap/src/wayland/pipewire.rs

@@ -276,12 +276,21 @@ impl PipeWireRecorder {
         // see: https://gitlab.freedesktop.org/pipewire/pipewire/-/issues/982
         src.set_property("always-copy", &true)?;
 
+        // COSMIC/Wayland fix: insert videoconvert between pipewiresrc and appsink.
+        // xdg-desktop-portal-cosmic's modifier negotiation fails when the downstream
+        // format set is too narrow (appsink only accepts BGRx/RGBx), producing
+        // "no more output formats" / not-negotiated (-4). videoconvert accepts any
+        // system-memory video/x-raw format, widening negotiation so the portal can
+        // settle on a format it can deliver via its SHM path.
+        let convert = gst::ElementFactory::make("videoconvert", None)?;
+
         let sink = gst::ElementFactory::make("appsink", None)?;
         sink.set_property("drop", &true)?;
         sink.set_property("max-buffers", &1u32)?;
 
-        pipeline.add_many(&[&src, &sink])?;
+        pipeline.add_many(&[&src, &convert, &sink])?;
-        src.link(&sink)?;
+        src.link(&convert)?;
+        convert.link(&sink)?;
 
         let appsink = sink
             .dynamic_cast::<AppSink>()