feat(node_assets): add image plane geometry nodes and validation scripts

- Implemented `image_plane.py` to build the Image Plane Geometry Nodes group with customizable parameters for geometry, subdivide, and thickness.
- Created `image_plane_validation.py` to validate the functionality of the image plane, ensuring correct geometry generation and material assignment.
- Developed `material_layers.py` to construct shader node groups for image materials, including depth and normal handling.
- Authored `plane_layouts.py` to define node arrangements for image geometry assets.
- Introduced `shadeless.py` to create a shadeless shader node group that displays color without scene lighting while preserving shadows and reflections.

tests/test_selection.py

@@ -1,7 +1,6 @@
 import json
 import sys
 from pathlib import Path
-
 import numpy as np
 import pytest
 
@@ -148,15 +147,6 @@ def test_scanline_matches_reference_for_random_paths():
             )
 
 
-def test_rasterizer_has_no_image_content_parameters():
-    import inspect
-
-    parameters = inspect.signature(selection.rasterize_selection_path).parameters
-
-    assert "source_alpha" not in parameters
-    assert "alpha_threshold" not in parameters
-
-
 def test_antialias_uses_only_padded_local_canvas(monkeypatch):
     selection_path = selection.SelectionPath(
         ((100, 200), (500, 200), (500, 600), (100, 600))