Rewrite spheric mirror to make it more efficient

[10110111]

The old version rewrote VBO about 150×3 times per frame, which is very inefficient, especially given that the data written never change. This version writes these constant data once in the constructor of the distorter and uses them on rendering.

Fixes #3535

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?

• Enabled spheric mirror distortion, checked performance, noted improvement in frame rate.
• Disabled distortion, checked that no errors are reported in the log.
• Enabled again, exited Stellarium, checked that no errors are reported.
• Run with --opengl-compat and with MESA_GL_VERSION_OVERRIDE=2.1 with distortion on, checked that the render looks as expected.
• Build against GLES2-using Qt6, check that in this mode distortion works as expected.

NOTE: I didn't try different parameters of the distortion, nor did I check the support for a custom distortion file. I don't expect that these would be affected, but this might be worth checking.

Test Configuration:

• Operating system: Ubuntu 20.04 LTS
• 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.

[gzotti]

YES!
Excellent performance now. (Win11/Geforce M3070)