Add ground projection support

[sebavan]

https://forum.babylonjs.com/t/projected-ground-skybox/40568

can be tested with https://playground.babylonjs.com/#XG08YC

related doc PR BabylonJS/Documentation#1000

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

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[Popov72]

Great!

[bjsplat]

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