Undistort masks alongside images in COLMAP workspaces - #4905

Closes #2361.

Add an optional mask_path to image_undistorter and pycolmap.undistort_images for COLMAP workspaces. Masks follow the exact output camera, crop, and resolution of their images, with nearest-neighbor sampling (including large downscales) and lossless grayscale PNG output under masks/<image_name>.png. Already-undistorted and spherical images are supported as well.

Accept the existing image-name-plus-.png convention and the extension-replacement fallback. Require readable, correctly sized masks for selected images; report errors through the controller/Python API and a failed CLI exit status. Other output formats explicitly reject mask_path. Existing calls without masks retain their behavior, and the new Python argument is keyword-only.

Validation on a CPU-only Release build (GUI/CUDA/MVS disabled):

• 39 C++ tests passed across controllers/undistorters_test, exe/image_test, image/undistortion_test, and image/warp_test.
• 11 Python tests passed in src/pycolmap/pipeline/images_test.py, including masked/unmasked calls and error propagation.
• CLI smoke test verified matching image/mask/model dimensions after downscaling and preservation of binary values; missing masks return exit status 1.
• clang-format 23.1.3, Ruff, and git diff --check passed.

Developed with assistance from OpenAI Codex.