oiiotool: Add --nchannels flag for parity with maketx

[grdanny]

Description

This PR adds a --nchannels flag to oiiotool, providing parity with the existing flag in maketx.

Problem:

When processing images from varied sources in automated pipelines, using name-based channel selection (e.g., --ch R,G,B) is fragile. If an input image uses non-standard names (e.g., X,Y,Z or 1,2,3), the command may output black channels or issue warnings that are easily missed in large logs.

Solution:

While you can solve this by specifiying the indices of the channnels, e.g --ch 0,1,2, the --nchannels flag offers a more intuitive and robust "shortcut" to force an image to a specific channel count. maketx has had one for years and I found that it works really well and often wished oiiotool would have one too.

Implementation:

The implementation acts as a wrapper around action_channels, programmatically generating the index string. It uses ustring to intern the generated string, ensuring memory safety even if the action execution is deferred/postponed.

Assisted-by: Google Gemini (advice on ustring interning and architectural structure).

Tests

I added some tests to showcase what this is trying to solve

Checklist:

• I have read the guidelines on contributions and code review procedures.
• I have read the Policy on AI Coding Assistants
  and if I used AI coding assistants, I have an Assisted-by: TOOL / MODEL
  line in the pull request description above.
• I have updated the documentation if my PR adds features or changes behavior.
• I am sure that this PR's changes are tested in the testsuite.
• I have run and passed the testsuite in CI before submitting the
  PR, by pushing the changes to my fork and seeing that the automated CI
  passed there. (Exceptions: If most tests pass and you can't figure out why
  the remaining ones fail, it's ok to submit the PR and ask for help. Or if
  any failures seem entirely unrelated to your change; sometimes things break
  on the GitHub runners.)
• My code follows the prevailing code style of this project and I
  fixed any problems reported by the clang-format CI test.
• If I added or modified a public C++ API call, I have also amended the
  corresponding Python bindings. If altering ImageBufAlgo functions, I also
  exposed the new functionality as oiiotool options.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: grdanny / name: Danny Greenstein (a418ed9)

[lgritz]

This LGTM, Danny, but it's marked as draft. Are you still working on it?

[grdanny]

This LGTM, Danny, but it's marked as draft. Are you still working on it?
Thank you!
Was just waitng for the CI to pass