provide virtual WebGL support

Plotly.js v2.28.0 added support for virtual-webgl, which allows you to put arbitrarily many WebGL-based graphs on a single page, as opposed to the limit of typically 4-8 before you hit the maximum contexts a browser will give you (note most desktop browsers give you up to 16 contexts, but one plotly.js graph uses up to 3 contexts). We would like to expose this option in Dash too. It can already be done by adding https://unpkg.com/virtual-webgl@1.0.6/src/virtual-webgl.js to external_scripts (or downloading that and putting it in your assets folder), so we could just document that, but it would be nicer to have a built-in way to do this.

Two important things to note about this:

• It's expected to have some performance impact, so if you only have one or two WebGL graphs at a time you're likely better off not using it, though we haven't quantified this.
• It takes over ALL WebGL on the page, not just plotly.js. This could be good, as you can then put an unlimited number of WebGL-based components of any type on the page, however we haven't tested it with other components so it's possible there will be issues. Aside from plotly.js (dcc.Graph), WebGL is used by some dash-bio components, dash-vtk, dash-deck, and possibly others (I don't think dash-cytoscape or dash-leaflet use WebGL but I'm not 100% sure of this).

Initially I was thinking we would make this opt-in as a prop of dcc.Graph, but because of the global (and irreversible) nature of this I'm now thinking a global setting like a Dash() constructor arg would be better.