[PBR][IBL] render an equirectangular image to cubemap

[bigbike]

Motivation and Context

As titled.
Given an HDRi image, we would like to use it as the "envionment" to light up the scene.
The 1st step is to render it to a cubemap.

This shader is to do it, and it will be used later in the PbrImageBasedLighting class.

How Has This Been Tested

Types of changes

• Docs change / refactoring / dependency upgrade
• 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)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.

[bigbike]

Picture is from: https://polyhaven.com/

CC0 license, completely free. see details here:
https://polyhaven.com/license

The cubemap results (Yes, they are up-side-down, which is totally CORRECT) :

All the functions are fully tested in the "ibl" branch.
see #1389