Add support transparency for landscapes

[alex-w]

Description

This pull request added support the transparency for landscapes.

Fixes #3214 (issue)

Previous solution is #3288

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

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]

Can you please explain the difference between this and the already reviewed working solution #3288 ? Why a new PR?

[alex-w]

Can you please explain the difference between this and the already reviewed working solution #3288 ? Why a new PR?

I renamed the branch to reflect logic of work and old PR was automatically closed.

[gzotti]

OK for me! The polygonal still looks bad in a few projections, but with the shader-based solution in the other landscapes this finally looks useful.

[github-actions]

Hello @alex-w!

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

[github-actions]

Hello @alex-w!

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