feat(heif): Monochrome channel read and write support, fix crash

[brechtvl]

Description

Implement support for reading and writing monochrome. Reading requires
libheif 1.17+ for heif_image_handle_get_preferred_decoding_colorspace.

Previously writing a single channel image would cause an exception due to
wrong parameters, but close() would continue writing the image and crash.
Destroy m_ctx on exception to prevent that for other potential errors.

Tests

Test added for monochrome read and write.

Checklist:

• I have read the guidelines on contributions and code review procedures.
• I have updated the documentation if my PR adds features or changes
  behavior.
• I am sure that this PR's changes are tested somewhere 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]

LGTM, and I'm going to amend a small docs fix if you don't mind.

[lgritz]

I took the liberty of pushing a minor documentation patch to your fork. Can you look it over and be sure it seems fine to you? Then I will merge.

[brechtvl]

Thanks, looks good.