VAT fails with ShaderMaterial
VAT fails with ShaderMaterial

VAT doesn't work with a ShaderMaterial, PG that reproduces the error: https://playground.babylonjs.com/#11GAIH#153

```
BJS - [17:55:14]: Error: VERTEX SHADER ERROR: 0:45: 'matricesIndices' : undeclared identifier
```

This is because `NUM_BONE_INFLUENCERS` is not defined. This patch defines it.
BabylonJS/Babylon.js,sebavan/Babylon.js,BabylonJS/Babylon.js,sebavan/Babylon.js,RaananW/Babylon.js,RaananW/Babylon.js,BabylonJS/Babylon.js,sebavan/Babylon.js,RaananW/Babylon.js