feat: offer both screens and windows in the Wayland portal picker

[B077AS]

Summary

Under Wayland the xdg-desktop-portal screen-cast dialog only offered the
source kind that matched the capturer we created: CreateScreenCapturer
showed just the monitor list, CreateWindowCapturer just the window list.
A user picking "share a window" from a screen-capture entry point (or vice
versa) had no way to get there.

This routes the Wayland/PipeWire path through
DesktopCapturer::CreateGenericCapturer, which requests
CaptureType::kAnyScreenContent — the same capture type Chromium uses for
getDisplayMedia. The portal then presents both the monitor and the
application-window tabs in a single dialog and the user chooses.

Details

• Only active when WEBRTC_USE_PIPEWIRE is defined.
• CreateGenericCapturer returns null when not running under Wayland or
  when PipeWire is disallowed; in that case the code falls back to the
  existing type-specific CreateWindowCapturer / CreateScreenCapturer
  based on sourceIsWindow, so X11, Windows and macOS behaviour is
  unchanged.
• The selected capturer is still wrapped in DesktopAndCursorComposer, as
  before.
• On the portal path the actual source selection is done by the portal
  dialog, so sourceIsWindow is not consulted there.

Files

• webrtc-jni/src/main/cpp/src/media/video/VideoTrackDesktopSource.cpp

Testing

• Wayland + PipeWire (GNOME): portal dialog now shows both "Entire Screen"
  and "Window" tabs; sharing a monitor and sharing a window both work.
• X11: unchanged, still uses the X11 screen/window capturers.
• Windows: unchanged (WGC path).