Cache UTC offset

[gzotti]

Finding UTC offset is surprisingly costly but is executed per-frame, shown in VTune. Caching the result per-minute saves at least part of it.

This needs some testing.

Description

Fixes # (issue)

Screenshots (if appropriate):

With this caching:

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] 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: Win11
• Graphics Card: irrelevant

Checklist:

• [x] My code follows the code style of this project.
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation (header file)
• [ ] I have updated the respective chapter in the Stellarium User Guide
• [x] 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[bot]]

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.

[10110111]

What is the performance benefit from this? Is it even measurable in any way except by using a profiler?

[gzotti]

What is the performance benefit from this? Is it even measurable in any way except by using a profiler?

I just wondered what takes so long just displaying InfoText. The commonInfoString was a surprisingly costly part of it, and here this seemingly trivial UTC lookup appears to be far from trivial.