Rasterise handles lists

[teunbrand]

Prompted by #25, this PR aims to allow list-input in the rasterise() function.

Regardless of whether the layer is geom_sf() or something else, handling lists is also convenient for rasterising multiple layers at once. With this PR, you can rasterise multiple layers:

library(ggplot2)
#> Warning: package 'ggplot2' was built under R version 4.0.3
library(ggrastr)

ggplot(mtcars, aes(wt, mpg)) +
  rasterise(list(
    geom_point(),
    geom_smooth()
  ))
#> `geom_smooth()` using method = 'loess' and formula 'y ~ x'

The updated rasterise() should be to find whatever layers are in a nested list, tag these for rasterisation, and ignore other list elements. In the geom_sf() case, this means that only the layer get rasterised, and not the coord. Below a demonstration for a nested list:

world1 <- sf::st_as_sf(maps::map('world', plot = FALSE, fill = TRUE))
ggplot() + rasterise(
  list(
    list(
      geom_sf(data = world1),
      theme(panel.background = element_rect(fill = "skyblue"))
    ),
    list(
      list(
        geom_point(aes(x = rnorm(100, sd = 10), y = rnorm(100, sd = 10)))
      ),
      theme(panel.border = element_rect(fill = NA, colour = "blue"))
    )
  )
)

The function still throws an error when provided a list that doesn't contain any layer elements. Note that I changed the error message as I found it confusing that it complained about the dev argument while the layer argument was at fault.

ggplot() + rasterise(
  list(scale_x_continuous(limits = c(-40, 40)))
)
#> Error: Cannot rasterise an object of class `list`.

^{Created on 2021-02-26 by the reprex package (v1.0.0)}

P.S.
I accidentally branched this from the master branch, which seems ahead of the develop branch. I'm sorry if this drags along unintended changes.

[evanbiederstedt]

Thanks @teunbrand

This does now handle the case mentioned in #25:

library(sf)
library(maps)
world1 <- sf::st_as_sf(maps::map('world', plot = FALSE, fill = TRUE))
library(ggplot2)
library(ggrastr)
ggplot2::ggplot() + ggrastr::rasterise(ggplot2::geom_sf(data = world1))

I accidentally branched this from the master branch, which seems ahead of the develop branch. I'm sorry if this drags along unintended changes.

No problem, the branches were pretty much in sync.

With this PR, you can rasterise multiple layers

I'll add to the vignettes, and mention this. Ditto with geom_sf() example.

[evanbiederstedt]

LGTM

Added an roxygen2 tag and added to the CHANGELOG.

I'll update the vignettes in the dev->master PR after the other PR is approved, and re-version.

Thanks!