Rename vViewPosition to vEyeDir

In the source code you use `V` which is usually used for View Vector aka Eye Direction.
Even the vertex shader calculates eye direction.

```
// eyeDir = cameraPosInViewSpace - viewPos
// eyeDir = vec3(0.0, 0.0, 0.0) - viewPos
// eyeDir = -viewPos

vViewPosition = -viewPos.xyz
```

https://github.com/glslify/glsl-perturb-normal/blob/master/demo/vert.glsl#L10