Add alphas channel for instanced meshes. Update example.

[Displee]

Related issue: https://discourse.threejs.org/t/add-opacity-to-vertice-colors-and-instanced-geometry/26298

Will give you the ability to set the opacity for colors in instanced meshes.

[WestLangley]

Transparency does not work well with instancing because the instances are not depth-sorted.

Here is the same scene rendered from different view directions:

[Displee]

I guess we need a separate example (or none at all) in that case.

[Mugen87]

We should not add features to the library that do not properly work. I vote to not support alpha for now.