Allow texture decimation

[gzotti]

Description

This is related to #2982. I want to make Stellarium again usable on a Raspberry Pi 3 which has significant problems with limited texture memory. This change allows to limit the texture size on loading the nebula catalog. It should be sufficient to show the "fuzzies" just with less resolution.

The changes were pretty involved, following the logic of the StelSkyImageTiles until finally loading the textures.
I failed to adapt the formulation for a template, Ruslan provided the solution.

Fixes #550 (issue)

Screenshots (if appropriate):

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

How Has This Been Tested?

Test Configurations:

• Win11/Geforce M3070Ti.
• Raspberry Pi 3

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?

Files matching guide/**:

• Did you remember to update screenshots to match new updates?
• Did you remember to grammar check in changed part of documentation?

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

[github-actions]

Hello @gzotti!

The enhancement or feature has been merged into source code and you may test it via building Stellarium from source code or wait the weekly development snapshot...

[github-actions]

Hello @gzotti!

The fix has been merged into source code and you may test it via building Stellarium from source code or wait the weekly development snapshot...

[github-actions]

Hello @gzotti!

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

[github-actions]

Hello @gzotti!

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