Add comprehensive tests for image processing and mask functionalities

- Introduced tests for color image handling, ensuring source preservation and premultiplied alpha usage.
- Implemented tests for brush stroke operations, validating segment boundaries and brush anti-aliasing.
- Added tests for frame anti-aliasing, confirming correct rendering behavior with various alpha values and depth settings.
- Created tests for frame projection, ensuring accurate overlap calculations and half-plane clipping.
- Developed tests for image premultiplied retention, verifying behavior during loading and packing processes.
- Added undo functionality tests for mask operations, ensuring single transaction behavior during edits.

tests/test_frame_projection.py

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+import sys
+from pathlib import Path
+
+import numpy as np
+import pytest
+
+sys.path.insert(0, str(Path(__file__).parents[1] / "src"))
+
+from anyimage.common.selection import clip_polygon_halfplanes
+from anyimage.operators.image_edit_tool import frame
+
+
+@pytest.mark.parametrize("focal", [1, 1.7, 2, 4])
+@pytest.mark.parametrize("angle,depth", [(30, 0.2), (0, 1e-14)])
+def test_frame_overlap_matches_finite_front_samples(focal, angle, depth):
+    projection = np.eye(4)
+    projection[0, 0] = projection[1, 1] = focal
+    projection[3] = (0, 0, -1, 0)
+    c, s = np.cos(np.radians(angle)), np.sin(np.radians(angle))
+    world = np.eye(4)
+    world[:3, :3] = ((c, 0, s), (0, 1, 0), (-s, 0, c))
+    world[2, 3] = -depth
+    source = frame.frame_source_projection(
+        projection, np.eye(4), world, (-1, 1, -1, 1), (100, 100), perspective=True
+    )
+    source["image_size"] = (100, 100)
+    assert frame.frame_source_coordinates(source, np.array(((50, 50),)))[2][0]
+    overlap = frame.frame_source_overlap(source, ((0, 100), (100, 100), (100, 0), (0, 0)))
+    assert len(overlap) >= 3
+    assert np.isfinite(overlap).all()
+
+
+@pytest.mark.parametrize("scale", [1e-14, 1, 1e14])
+def test_halfplane_clipping_is_scale_invariant(scale):
+    square = ((0, 0), (2, 0), (2, 2), (0, 2))
+    result = clip_polygon_halfplanes(square, ((-scale, 0, scale),))
+    assert set(result) == {(0, 0), (1, 0), (1, 2), (0, 2)}
+    assert clip_polygon_halfplanes(square, ((1, 0, -3),)) == ()
+    edge = clip_polygon_halfplanes(square, ((1, 0, -2),))
+    assert all(x == 2 for x, y in edge)