Add option to use virtual-webgl for handling multiple WebGL contexts

[archmoj]

@plotly/plotly_js

Resolves #2333 for parcoords, gl2d, gl3d and mapbox subplots thanks to @greggman's great work on https://github.com/greggman/virtual-webgl

TODOs:

• Fix case of scattergl.mode='line' not initially show lines
• Add documentation, users need to include following script on their page before other scripts:

<script src="https://unpkg.com/browse/virtual-webgl@1.0.6/src/virtual-webgl.js"></script>

[archmoj]

Main fix was here: plotly/mapbox-gl-js@71e2cfc

[alexcjohnson]

@archmoj this looks great, really nice work! No comments, other than as discussed let's try to skirt the only-lines issue by adding mock markers - either tiny and matching the line color, or maybe they can be specified as transparent... or maybe there's a way to add markers that doesn't require them to match the input arrays at all, and we can just add a single marker at some point that we know won't affect anything? However you come up with to do it, make sure it doesn't impact autorange.

[alexcjohnson]

Excellent, you found a way to fix lines-only that doesn't involve the phantom markers hack 🎉

💃