[2021.2] Fix shadow rendering correctly to work with shader stripping in WebGl

[lukaschod]

Purpose of this PR

Backport #6300

[github-actions]

Hi! This comment will help you figure out which jobs to run before merging your PR. The suggestions are dynamic based on what files you have changed.
Link to Yamato: https://unity-ci.cds.internal.unity3d.com/project/902/
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URP
/jobDefinition/.yamato%2Fall-urp.yml%23PR_URP_2021.2
With changes to URP packages, you should also run
/jobDefinition/.yamato%2Fall-lightmapping.yml%23PR_Lightmapping_2021.2

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[github-actions]

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[lukaschod]

Build URP_PostPro on iPhone_Metal_il2cpp_Linear_Standalone_build_Player on version
https://unity-ci.cds.internal.unity3d.com/job/9989413
exitCode: 138

ShaderGraph on Linux_Vulkan_Standalone_mono_Linear on version 2021.2
ShaderGraph on Win_DX11_Standalone_mono_Linear on version 2021.2
https://unity-ci.cds.internal.unity3d.com/job/10078725
ShaderGraphGraphicsTests.MathNodes

ShaderGraph on OSX_Metal_playmode_mono_Linear on version 2021.2
https://unity-ci.cds.internal.unity3d.com/job/10043234
Instability

URP_Terrain on Android_Vulkan_Standalone_il2cpp_Linear on version 2021.2
https://unity-ci.cds.internal.unity3d.com/job/9997330
UniversalGraphicsTests.035_Shader_TerrainShaders

No new failures, will wait for #6300

[ernestasKupciunas]

Looks good. The issue is fixed. Verified using the repro steps written in FB case(https://fogbugz.unity3d.com/f/cases/1381881/).
@lukaschod since this is a regression, please do a root cause analysis.

[lukaschod]

@ernestasKupciunas

What is the current workflow for root cause analysis? I remember it changed.