Fix broken prefer_canvas option

[FabeG]

Small correction to fix broken prefer_canvas option (Close #1131):
since leaflet 1.0.0, the preferCanvas option is not part of the GlobalSwitch
options, but has to be passed as an option to L.Map.

[Conengmo]

Thanks for making a PR for this! Seems we indeed missed the change on the Leaflet side.

I have one comment, once that's resolved this is good to merge.

[Conengmo]

This looks good @FabeG, I’ll merge it later this week.

Can I ask, maybe you know, why wouldn’t we always use prefer_canvas? If it increases performance we could set the default to true. Or are there downsides or side effects?

[FabeG]

Hi @Conengmo, I let the default to false to be in line with leaflet default. It seems there are still some drawbacks using Canvas as a default. See https://leafletjs.com/reference-1.0.0.html#canvas :

Canvas is not available in all web browsers, notably IE8, and overlapping geometries might not display properly in some edge cases

[Conengmo]

Thanks for looking into this. We'll leave it at default False then.

This is ready IMO, I'll merge it later this week. Thanks for your contribution!