Implement ext-image-capture-source and ext-image-copy-capture protocols (staging)

[powersemmi]

Implement the following Wayland protocols from the ext-image-capture-source and ext-image-copy-capture staging protocol families:

Protocol	Description	Status
ext_image_capture_source_v1	Opaque image capture source objects	Added via smithay's ImageCaptureSourceState
ext_output_image_capture_source_manager_v1	Create capture sources from outputs	Added via smithay's OutputCaptureSourceState
ext_foreign_toplevel_image_capture_source_manager_v1	Create capture sources from toplevels	Custom implementation for niri's foreign_toplevel
ext_image_copy_capture_v1	Capture image content into SHM/DMABuf buffers	Full rendering support
ext_foreign_toplevel_list_v1	List of toplevel windows	Already implemented (added public lookup method)

Architecture

Capture Sources

• Output sources: store WeakOutput in the source's user_data. Created via ext_output_image_capture_source_manager_v1.
• Toplevel sources: store WlSurface in the source's user_data. Created via ext_foreign_toplevel_image_capture_source_manager_v1, with a custom handler that looks up the window by its ExtForeignToplevelHandleV1 through niri's existing ForeignToplevelManagerState.

Copy Capture

• ImageCopyCaptureState registered as a global.
• capture_constraints() returns the output or window size plus supported SHM formats (Argb8888, Xrgb8888).
• frame() renders the captured content into the client-provided buffer:
  • Output capture: renders the full output via Niri::render() with RenderTarget::ScreenCapture.
  • Toplevel capture: finds the mapped window via find_root_shell_surface() + Layout::find_window_and_output(), renders via Mapped::render_for_screen_cast().
  • Supports both SHM buffers (via render_to_shm) and DMA-BUF buffers (via render_to_dmabuf).
  • Uses OutputDamageTracker with full damage (age=0) for correct sizing and rendering.
  • Signals completion with frame.success().

Toplevel Manager

Since niri implements ext_foreign_toplevel_list_v1 with its own state (not smithay's ForeignToplevelHandle), a custom ToplevelImageCaptureManagerState was implemented that:

1. Registers the ExtForeignToplevelImageCaptureSourceManagerV1 global.
2. On CreateSource, looks up the ExtForeignToplevelHandleV1 in ForeignToplevelManagerState::find_surface_for_handle() to find the corresponding WlSurface.
3. Creates an ImageCaptureSource and stores the WlSurface in its user_data.

Notes

• The ext_foreign_toplevel_list_v1 protocol was already fully implemented. A new find_surface_for_handle() method was added to ForeignToplevelManagerState for the toplevel capture source manager.
• Frame rendering is synchronous within the Wayland dispatch handler, using self.backend.with_primary_renderer() to access the GlesRenderer.

Closes #1558

[powersemmi]

Hi! I'd appreciate some help from the maintainer with reviewing and
possibly refactoring this PR.

A few things I'm unsure about and would love feedback on:

1. The ToplevelImageCaptureManagerState in
   src/protocols/toplevel_image_capture_source.rs — since niri has its own
   ext_foreign_toplevel_list_v1 implementation (not using smithay's
   ForeignToplevelHandle), I wrote a custom handler. It works, but maybe there's a
   cleaner way to integrate it with the existing foreign_toplevel state.

2. The synchronous rendering in ImageCopyCaptureHandler::frame() — currently it
   renders directly in the Wayland dispatch handler via
   self.backend.with_primary_renderer(). This might have performance implications or
   issues with renderer state. Should the rendering be deferred to the next output
   redraw instead?

3. The frame constraints and rendering for toplevel/window capture — I'm computing
   the window bbox size on the fly, but maybe this should be cached or handled
   differently.

Also, a heads-up: this PR was primarily created to test my tool
RemoteWay — a remote desktop solution
that uses these protocols. So while the implementation works for the basic use case,
it could definitely use a proper review and cleanup from someone more familiar with
niri's internals.

Any guidance would be greatly appreciated! 🙏