feat(oiiotool): Add thumbnail get/set commands and fix TGA thumbnail I/O

[jinhgkim]

Description

Adds oiiotool commands to extract and attach embedded thumbnails, and fixes TGA postage-stamp read/write bugs they exposed. This PR adresses #4889.

Note that thumbnail support is uneven across formats: get_thumbnail is implemented for PSD, camera RAW, and TGA, but set_thumbnail only for TGA. So you can read thumbnails from several formats, yet TGA is the only format that writes an embedded thumbnail today. e.g., input.exr ... --set-thumbnail -o out.exr silently produces an EXR with no embedded thumbnail.

• Features:

  • oiiotool: add --get-thumbnail
  • oiiotool: add -i:get_thumbnail=1
  • oiiotool: add --set-thumbnail

• Fixes:

  • targa thumbnail write: encode thumbnail like the main image (bottom-up, deassociated alpha, BGR(A))
  • targa thumbnail read: apply associateAlpha()

Assisted-by: Claude Code / opus-4.8

Tests

New tests in oiiotool-get-thumbnail, oiiotool-set-thumbnail, imagebuf_test

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: jinhgkim / name: Jinnie Kim (2c43c72, 4a1c708, 54e6a14, 685c519, 6caac64, da57681, da95bb8)
• ✅ login: jinhgkim / name: jinniekim (797f600)

[jinhgkim]

During the CI run, a TGA thumbnail test failed because I added associateAlpha() logic in this branch. My reasoning is that if we apply associateAlpha() in TGAInput::readimg(), we should apply the same operation when reading thumbnails.

Let me know what you think.

Also, fyi, I have broken down the commits into logical steps with some details in the commit message.