Automatically set aria:false if the renderer is canvas

[kanitw]

We should automatically set aria: false if the renderer is canvas since aria descriptions are not used on canvas (they only make sense for SVG), and they can be expensive to compute.

[domoritz]

Why should embed do this and not Vega or Vega-Lite? I see that the information is here and not in Vega-Lite but it feels odd.