Don't modify legend key settings in geom_sf() during plot build.

[clauswilke]

Fixes #3941.

library(ggplot2)

nc <- sf::st_read(system.file("shape/nc.shp", package = "sf"), quiet = TRUE)
p <- ggplot(nc[1:3, ]) +
  geom_sf(aes(color = NAME), show.legend = "point")
p

p

^{Created on 2020-04-11 by the reprex package (v0.3.0)}

[yutannihilation]

Looks good, but as I don't quite remember the discussion on #3646, l need some more time to see the context...

[yutannihilation]

After reading the comments around #3646 (comment), I remembered I once proposed a similar approach, but it uses param$legend directly. Here's a version for this PR:

yutannihilation@2b6317d

Do you think we need to introduce a new parameter on LayerSf rather than recycling legend (or some other parameters in geom_params)?

[clauswilke]

I think writing to params$legend directly can create a warning of an unknown parameter if you're using the layer with a geom that doesn't have a legend parameter.

[yutannihilation]

I see. So, can we do this in geom_sf()?

[yutannihilation]

I mean, if this is specific to geoms that have a legend parameter, it makes sense to do it in geom_sf(). If this is more general one, this should be done in layer_sf(). (And I'm getting to feel the latter is right...)

[yutannihilation]

After some thinking, I'm sure this is the best fix at the moment.