[testrender] Enable mipmap in optix by switching to cached ImageBuf

[AlexeySmolenchuk]

Description

This PR workarounds this issue

I also increased maxAnisotropy to the maximum to prevent overblurring and match CPU results better.

Tests

Expect some Optix tests may fail.

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.

[lgritz]

I think something is going wrong, see the logs in the failing optix version.

[AlexeySmolenchuk]

Turns out the cached version is also buggy.
I've updated OIIO issue.
I'll replace it with a more reliable seek_subimage()

[AlexeySmolenchuk]

Test
render-background	Hdri image now filtered anisotropically
render-raytypes	Hdri image now filtered anisotropically
testoptix-noise	Seems refimage for gabor noise is outdated. It doesn't match even without this commit.
render-spi-thinlayer	I can see "an illegal memory access" error in all recent CI, even successful