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.

src/anyimage/server/geometry/normal_texture.py

@@ -40,15 +40,9 @@ def write_camera_normal_texture(
     if visibility.shape != normal.shape[:2]:
         visibility = resized_visibility(visibility, normal.shape[:2])
     normal_mode = str(normal_mode).upper()
-    if normal_mode == "TANGENT":
+    if normal_mode in {"TANGENT", "OBJECT"}:
         converted = normal * np.asarray((1.0, -1.0, -1.0), dtype=np.float32)
         empty_normal = (128, 128, 255)
-    elif normal_mode == "OBJECT":
-        converted = normal[..., (2, 0, 1)] * np.asarray(
-            (1.0, -1.0, -1.0),
-            dtype=np.float32,
-        )
-        empty_normal = (0, 128, 128)
     else:
         raise ValueError(f"Unsupported normal mode: {normal_mode}")
     encoded = np.clip(