Make rectangular FoV marking follow real projection

[10110111]

Fixes #3015

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 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]

Oh, thanks for quick solution!

The fix is not changed translatable strings, so, we can accept it for release 23.2 - is it OK for all?

[gzotti]

@alex-w shall this be permanently deleted?

[alex-w]

It's a "hidden feature" :) As far nobody asked for it before, so, this part may be removed.

[gzotti]

When changing to equatorial (telescope) mode, the box is now tilted. @alex-w is this what you wanted?

[alex-w]

When changing to equatorial (telescope) mode, the box is now tilted. @alex-w is this what you wanted?

This is good question. Previous solution didn't follow the mount types, but the current solution is followed it (derotate parameter). I think now it's better.

[gzotti]

I am less sure about that and would see using the current mode more intuitive. However, if you like it being always in Azimuthal mode, it's OK to merge. I only use the Ocular plugin's sensor frame.