OutlinePass: change var to const

[tomhsiao1260]

I think it's better to use ES6 const syntax instead of var in this case. So I send a PR for this example.

[mrdoob]

How come you removed these? 🤔

[tomhsiao1260]

this.maskBufferMaterial is unused in this example which makes me confused for a while. So in my opinion, I think it's more reasonable to remove it.

[mrdoob]

Any idea of why that code was there to begin with?
You can look at the commit history to investigate.

[tomhsiao1260]

I've checked the history and found it begin with the initial commit and this.maskBufferMaterial is also unused in the initial version.

Here's the link: 9e6f430#diff-9b3033a5e289a75f2da1383ac047c096c8737a31f3d70ccdeebd2b6d2faf3dcdR48

[tomhsiao1260]

Since this.maskBufferMaterial is removed, we don't need to import MeshBasicMaterial. So I remove it as well.

[mrdoob]

Thanks!