Variable sizes for geom_hex()

[teunbrand]

This PR aims to fix #6727.

It introduces the new radius aesthetic for geom_hex(), which let's us declare a relative size of the hexagon.

devtools::load_all("~/packages/ggplot2/")
ggplot(faithful, aes(waiting, eruptions)) +
  geom_hex(
    aes(radius = after_stat(ncount)), 
    fill = "black"
  )

^{Created on 2025-11-05 with reprex v2.1.1}

I decided that this radius aesthetic it doesn't need a legend representation, because neither have width or height which are similarly position-related non-position aesthetics.

Draw method is faster now.

library(ggplot2)
p <- ggplot(diamonds, aes(carat, price)) +
  geom_hex(binwidth = c(0.05, 100))
b <- ggplot_build(p)
bench::mark(get_layer_grob(b), min_iterations = 100) # CRAN version
#> # A tibble: 1 × 6
#>   expression             min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr>        <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 get_layer_grob(b)   5.36ms   5.82ms      169.    5.06MB     27.6

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2
p <- ggplot(diamonds, aes(carat, price)) +
  geom_hex(binwidth = c(0.05, 100))
b <- ggplot_build(p)
bench::mark(get_layer_grob(b), min_iterations = 100) # This PR
#> # A tibble: 1 × 6
#>   expression             min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr>        <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 get_layer_grob(b)   1.72ms   1.83ms      463.    4.23MB     90.2

^{Created on 2025-11-05 with reprex v2.1.1}