Limit amount of time taken by texture loading per frame

[10110111]

Description

Previously (#3148) we thought that it was file I/O that lead to stalls during texture loading. It appears that it is I/O, but not a file one. Instead it's the calls to glTexImage2D that have to happen in the main thread, and that are done on first successful StelTexture::bind call. This PR reduces total time per frame taken by such texture uploads, deferring them to subsequent frames when this time is used up.

This lets us have smoother performance while not even having to limit the amount of threads that load and decode texture data. This PR also lifts this limitation on the number of threads (leaving the pool size still equal to the "ideal thread count").

Please keep the commits separated, just in case a need to revert one of them arises.

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: Ubuntu 20.04
• Graphics Card: Intel UHD Graphics 620

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.

[alex-w]

Yes, it better now. Thanks!

[gzotti]

Very smooth now, thanks!

[github-actions]

Hello @10110111!

Please check the fresh version (development snapshot) of Stellarium:
https://github.com/Stellarium/stellarium-data/releases/tag/weekly-snapshot

[github-actions]

Hello @10110111!

Please check the latest stable version of Stellarium:
https://github.com/Stellarium/stellarium/releases/latest