mesh3d transparency not activated by color alpha channel 

This `mesh3d` bug is similar to what was reported by @jonmmease on `scatter3d` plots https://github.com/plotly/plotly.js/issues/3492; but this is related to `gl-mesh3d`
[Codepen](https://codepen.io/MojtabaSamimi/pen/XQNdZr?editors=0010).

