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_image_premul_retention.py

@@ -0,0 +1,50 @@
+from io import BytesIO
+
+import bpy
+import numpy as np
+import pytest
+from PIL import Image
+
+
+@pytest.mark.parametrize("loaded_png", [False, True])
+@pytest.mark.parametrize("mode_before_pack", [False, True])
+def test_premul_byte_reload_requires_straight_for_original_rgb(
+    tmp_path, loaded_png, mode_before_pack
+):
+    """Characterize the reload mismatch blocking automatic PREMUL defaults."""
+    rgba = np.array(
+        [[51, 102, 204, alpha] for alpha in (0, 64, 128, 255)], dtype=np.uint8
+    ).reshape(1, 4, 4)
+    expected = rgba.reshape(4, 4) / 255
+    path = tmp_path / "source.png"
+    Image.fromarray(rgba).save(path)
+    if loaded_png:
+        image = bpy.data.images.load(str(path), check_existing=False)
+    else:
+        image = bpy.data.images.new("Premul probe", width=4, height=1, alpha=True)
+        image.pixels.foreach_set(expected.astype(np.float32).ravel())
+    image.use_fake_user = True
+    image.alpha_mode = "STRAIGHT"
+    if mode_before_pack:
+        image.alpha_mode = "PREMUL"
+    image.pack()
+    image.alpha_mode = "PREMUL"
+    packed = np.asarray(Image.open(BytesIO(image.packed_file.data)))
+    np.testing.assert_array_equal(packed, rgba)
+    name = image.name
+    blend_path = str(tmp_path / "retention.blend")
+    bpy.ops.wm.save_as_mainfile(filepath=blend_path)
+    bpy.ops.wm.open_mainfile(filepath=blend_path)
+    restored = bpy.data.images[name]
+    assert restored.alpha_mode == "PREMUL"
+    interpreted = np.array(restored.pixels[:]).reshape(4, 4)
+    np.testing.assert_allclose(interpreted[0, :3], 1)
+    assert not np.allclose(interpreted, expected, atol=1 / 255)
+    restored.alpha_mode = "STRAIGHT"
+    np.testing.assert_allclose(
+        np.array(restored.pixels[:]).reshape(4, 4), expected, atol=1 / 255
+    )
+    restored.alpha_mode = "PREMUL"
+    np.testing.assert_allclose(
+        np.array(restored.pixels[:]).reshape(4, 4), interpreted, atol=1 / 255
+    )