Fix material array handling in shadow component

[takahirox]

Description:

This PR fixes material array handling in shadow component.

In Three.js r89, mesh.material can be THREE.*Material or array of THREE.*Material.

THREE.MultiMaterial is already deprecated, it's converted to material array.

So we should check if mesh.material is array instead.

Question: Is var name meshWithMaterialArray in the test too long?

Changes proposed:

• use node.material instead of node.material.materials for material array in shadow component.