[Fix] Honor channel order for RGB TIFF images

[tandede]

Motivation

Fixes #1101.

The tifffile backend returns standard color TIFF data in native RGB order, but MMCV's documented default is BGR. The backend previously returned the raw array without consulting channel_order, so default reads displayed red and blue channels incorrectly.

Modification

• Read TIFF metadata to identify images whose photometric interpretation is RGB and locate their sample axis.
• Reorder the first three samples for BGR output while preserving alpha or other extra samples and supporting both contiguous and planar layouts.
• Keep non-RGB multi-channel TIFF data unchanged, preserving the backend's remote-sensing use case.
• Add regression coverage for RGB, RGBA, planar RGB, explicit RGB output, and the existing five-channel non-RGB fixture.

BC-breaking (Optional)

No. This makes the tifffile backend honor the existing channel_order contract. Callers that need native RGB output can continue to pass channel_order='rgb' explicitly.

Validation

• pytest -q tests/test_image (61 passed)
• Flake8, isort, YAPF, codespell, and whitespace checks pass for both changed files.
• No documentation update is needed because this restores the documented channel_order behavior.

[CLAassistant]

All committers have signed the CLA.

[tandede]

Hi, I checked the current failed statuses. The CircleCI pipeline stops during configuration fetch with “No configuration was found in your project,” while both Read the Docs builds stop during configuration validation because sphinx.configuration is missing. None of these jobs reaches the TIFF implementation or its tests, so they appear to be repository CI configuration issues. Could a maintainer rerun or adjust these checks when convenient? Thanks!