Using after_scale() in Geom$default_aes field

[teunbrand]

This PR aims to fix #6135.

It works by appending any scaled/staged expressions in Geom$default_aes to the modified computed mapping and evaluating these along with them.

Reprex from issue:

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

GeomPointAlt <- ggproto(
  "GeomPointAlt", GeomPoint,
  default_aes = aes(
    colour = after_scale(alpha(fill, 0.4)),
    fill = "black",
    # Merge with default, removing pre-existing `colour`/`fill`
    !!!utils::modifyList(GeomPoint$default_aes, list(colour = NULL, fill = NULL))
  )
)

The issue exposed in #6135 (comment), note that colour now has an alpha channel.

get_geom_defaults(GeomPointAlt)
#>      colour shape size alpha stroke  fill
#> 1 #00000066    19  1.5    NA    0.5 black

Display is identical to desired plot in issue:

ggplot(mpg, aes(displ, hwy, fill = drv)) +
  stat_identity(geom = GeomPointAlt)

^{Created on 2024-10-15 with reprex v2.1.1}

[thomasp85]

LGTM