[imageio] Support TIFF with any number of samples per pixel

[victoryforce]

Fixes #18187.

This PR fixes a regression introduced in 5.0.0 after the imageio code refactoring.

Actually, the TIFF loader itself did not support reading 2-channel (luminance + alpha) images before either. But darktable is usually built with support for the GraphicsMagick (or ImageMagick) fallback loader. So such files used to be loaded using a fallback loader.

This broke when additional loader failure codes were introduced, and the code returned in this particular case (DT_IMAGEIO_UNSUPPORTED_FEATURE) was not added to the fallback loader call condition.

The TIFF loader code was actually ready to correctly read files with any number of channels (samples per pixel). So this fix simply removes the restriction on the number of channels and adds the correct interpretation of 2-channel files as monochrome.

[TurboGit]

Merged in master and 5.0.x branch.

[victoryforce]

Files to verify the correctness of this fix can be downloaded from https://github.com/tlnagy/exampletiffs:

• house.tif - 2 channel (luminance + alpha) image
• shapes_hyper.tif - 7 channels (RGB + 4 extra channels that we have to ignore)

These files must be read correctly, the "image information" module must specify TIFF loader (not fallback one, like GraphicsMagick).

[victoryforce]

Suggested release notes text:

Fixed a regression introduced in 5.0.0 where we lost the ability to read 2-channel TIFF files and files with more than 4 channels. This fix allows them to be read by the TIFF loader, whereas previously the TIFF loader rejected such files and they were loaded by a fallback loader (GraphicsMagick or ImageMagick).

[TurboGit]

Thanks!