Handle when geometry is not a symbol

[yutannihilation]

Fix #4122

Currently, LayerSf$setup_layer() uses as_name(self$mapping$geometry) in the assumption that geometry is a quosure of a symbol, which is not always the case. Possible fix would be

1. actually evaluate it
2. give up guessing the type nicely

and I chose option 2. because option 1. seems too much for this rare use case.

devtools::load_all("~/repo/ggplot2")
#> Loading ggplot2

nc <- sf::st_read(system.file("shape/nc.shp", package = "sf"), quiet = TRUE)
ggplot(nc) +
  geom_sf(aes(fill = AREA, geometry = sf::st_centroid(geometry)))
#> Warning in st_centroid.sfc(geometry): st_centroid does not give correct
#> centroids for longitude/latitude data

#> Warning in st_centroid.sfc(geometry): st_centroid does not give correct
#> centroids for longitude/latitude data

^{Created on 2020-07-10 by the reprex package (v0.3.0)}

[clauswilke]

Yes, this makes sense.

[yutannihilation]

Thanks.