WebGLShadowMap: Properly honor customDepthMaterial

[WestLangley]

Replacement PR for #25082

[WestLangley]

@titansoftime Can you please try this with both your customDepthMaterial version and your InstancedMesh version and report back?

[titansoftime]

Looks good! Tested with both CSM and regular ShadowMaps with instancing done both by InstancedMesh and building from instancedGeometry and extending shaders.

I must note, I do wonder if the actual customDepthMaterial is what is being used though as my customDepthMaterial previously yielded imperfect shadows (applied to full mesh even though mesh consisted of parts, some alphaTest, some not). Now, they look great using either method. Looks great though. Thank you.

Wolfie thanks you too. He likes the shade.