Fix/softimage channel bounds

[ssh4net]

Description

Fixes malformed Softimage PIC channel packet handling.

Before this change, the reader counted the number of channels in the file, but decoded pixels using fixed channel indexes: R=0, G=1, B=2, A=3. A malformed PIC could advertise only an alpha channel, producing a one-channel ImageSpec, while decode still wrote to channel index 3. That could write past the scanline
buffer in iconvert, iinfo --stats, or other callers that read pixel data from an untrusted .pic.

After this change, Softimage channel packets are validated during open() before the output spec is accepted. The reader now rejects empty, duplicate, unsupported, mixed-depth, and sparse channel packet maps instead of accepting a layout that decode cannot safely write into the declared channel count.

Tests

Added regression coverage to testsuite/softimage.

The test now generates small malformed PIC files with alpha-only sparse channel packets for the uncompressed, pure-RLE, and mixed-RLE decode paths. Each file is passed through iconvert to out.null, and the test records a rejection marker when the malformed input is rejected.

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: Codex GPT5.5 xHigh
  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.

[lgritz]

Can you rebase this one on top of current main? I think it now conflicts slightly with the other I just merged.