Adapt to new theming in ggplot2

[teunbrand]

This PR aims to fix an issue identified in tidyverse/ggplot2#6287.

Briefly, it incorporates recent changes to ggplot2 themes into gganimate's build method.

As an example (I'm sorry for the default windows GD 😁):

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

p <- ggplot(iris, aes(x = Petal.Width, y = Petal.Length, size = Sepal.Width)) + 
  geom_point() +
  theme_gray(ink = "white", paper = "black")

anim <- p + 
  transition_states(Species,
                    transition_length = 2,
                    state_length = 1)

anim

^{Created on 2025-01-23 with reprex v2.1.1}

[teunbrand]

There is still the matter of labels. ggplot2 no longer precomputes these up-front, but uses ggplot2:::setup_plot_labels() to do this during the build stage. It is not a straightforward function to copy to gganimate as it reaches deep within ggplot2's internals.