Treat LoS special case of exactly on terrain as visible

[Ryanf55]

What does this PR do?

Treat the special case of requesting line of sight from above to a point exactly at the same altitude as the DEM as visible.

That change breaks another test, I'll fix it then move this out of draft.

What are related issues/pull requests?

Closes #12458

Tasklist

• [ ] AI (Copilot or something similar) supported my development of this PR
• [ ] Make sure code is correctly formatted (cf pre-commit configuration)
• [ ] Add test case(s)
• [ ] Add documentation
• [ ] Updated Python API documentation (swig/include/python/docs/)
• [ ] Review
• [ ] Adjust for comments
• [ ] All CI builds and checks have passed
• [ ] ADD YOUR TASKS HERE

Environment

Provide environment details, if relevant:

• OS: Ubuntu 22
• Compiler: GCC 11

[rouault]

GDALIsLineOfSightVisible_through_mountain needs to be adjusted and code formatting issue (https://github.com/OSGeo/gdal/actions/runs/15240459629/job/42859870241?pr=12460)

[Ryanf55]

GDALIsLineOfSightVisible_through_mountain needs to be adjusted and code formatting issue (https://github.com/OSGeo/gdal/actions/runs/15240459629/job/42859870241?pr=12460)

I am aware, I'll chase the failure down shortly after the holiday.

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