Terrain material: Fix bug when using Cascaded Shadow Maps

[Popov72]

See https://forum.babylonjs.com/t/terrain-self-shadow-with-cascadedshadowgenerator/54820

[bjsplat]

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To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

Snapshot stored with reference name:
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Test environment:
https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/refs/pull/15841/merge/index.html

To test a playground add it to the URL, for example:

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[bjsplat]

Visualization tests for WebGPU (Experimental)

https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/refs/pull/15841/merge/testResults/webgpuplaywright/index.html

[bjsplat]

WebGL2 visualization test reporter:

https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/refs/pull/15841/merge/testResults/webgl2playwright/index.html