Add padding to bounding cap to avoid clipping on small FOV

[henrysky]

Add padding to bounding cap to avoid clipping on small FOV. The root cause is Saemundsson's inversion formula for atmospheric refraction is not exact as stated in

stellarium/src/core/RefractionExtinction.cpp

Line 253 in 8a129b6

//Bennett's formula is not a strict inverse of Saemundsson's. There is a notable discrepancy (alt!=backward(forward(alt))) for

0.5 arcmin padding seems to work well except a few arcminute above the horizon (which is really close and I think this is good enough).

Fixes #365, #538

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• This change requires a documentation update
• Housekeeping

How Has This Been Tested?

Test Configuration:

• Operating system: MacOS 15
• Graphics Card: Apple M3

Checklist:

• My code follows the code style of this project.
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (header file)
• I have updated the respective chapter in the Stellarium User Guide
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[github-actions]

Great PR! Please pay attention to the following items before merging:

Files matching src/**/*.cpp:

• Are possibly unused includes removed?

This is an automatically generated QA checklist based on modified files.

[gzotti]

Ah, fresh eyes on an old problem! Looks like you had the right idea. Great, thanks!