<AsIs> never invokes identity scale

[teunbrand]

This PR to the RC branch aims to fix #5596.

Briefly, it early-exits find_scale() when x is an <AsIs> object.

Consequently, no identity scale is added in ScalesList$add_defaults and <AsIs> aesthetics can be combined with regular aesthetics again over different layers. For good measure, the scale_type.AsIs method was removed too.

The reprex from #5596 now gives the same plot regardless of layer order:

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

library(ggplot2)

df <- data.frame(
  x = 1:4, y = 1:4,
  colour = c("red", "green", "blue", "yellow"),
  group  = LETTERS[1:4]
)

ggplot(df, aes(x, y)) +
  geom_text(aes(label = group, colour = group)) +
  geom_text(aes(x = 5 - x, label = colour, colour = I(colour)))

ggplot(df, aes(x, y)) +
  geom_text(aes(x = 5 - x, label = colour, colour = I(colour))) +
  geom_text(aes(label = group, colour = group))

^{Created on 2023-12-18 with reprex v2.0.2}

[thomasp85]

LGTM

[teunbrand]

Thanks!