plot_njt() fails to render on some browser/driver combos

[cclarkson]

I am finding that larger NJTs (multi-country) wouldn't render on my machine (Windows, Firefox). I would get a message saying "WebGL is not supported by your browser", however WebGL is supported and configured correctly. Plots with fewer samples rendered fine.

I could get the larger plots to render by using render_mode="svg". This is the default for plot_haplotype_networks(), perhaps a similar issue was found there?

We could change plot_njt() to match plot_haplotype_networks?