WebGL context attributes

The answer to my question about setting a transparent background for a WebGL canvas ended up being pretty straightforward.

A WebGL context can be configured and one prarmeters is

alpha: Boolean that indicates if the canvas contains an alpha buffer.

from developer.mozilla.org

In this case, the tools I am using had hidden this option and had turning it off, no alpha.

I'll look at offering a PR back to OpenJSCAD now.