Static image export in Deno for VlConvert

[jonmmease]

Cross reference vega/vl-convert#24

Down the road, I'm interested in using the Vega WebGPU renderer to accelerate PNG export of charts with lots and lots of marks (e.g. scatter plots with a million points). Deno has initial support for WebGPU, which is something that even nodejs/node#41994.

If we could get the WebGPU renderer working in Deno, I think we could use it for accelerated PNG image export in VlConvert. I haven't tried them yet, but here are some examples of WebGPU support in Deno.

Since browsers don't support WebGPU out of the box yet, including this renderer (even as en experimental mode) in VlConvert might be a good way to get it into the hands of users in the meantime.

Running in Deno might also be a nice way to create a CI test suite.