gganimate ignores font face in theme()

[danm0nster]

Thanks for a wonderful package! I think I found a bug, but, of course, it could also just be me doing something wrong.

Description of the issue: When adding animations to a plot where the fonts have been changed from the default, gganimate seems to ignore face = "italic" in the call to theme().

Minimal reproducible example: Below is a code snippet with a base plot where x and y labels are correctly italicised, but adding an animation changes the label font face to normal.

library(ggplot2)
library(gganimate)

base_plot <- ggplot(
  data.frame(x = c(1, 4), y = c(4, 1)),
  aes(x = x, y = y)
) +
  geom_point() +
  expand_limits(x = c(0, 5), y = c(0, 5)) +
  coord_fixed(ratio = 1) +
  theme_classic() +
  theme(
    axis.text = element_text(size = 12, family = "Helvetica", face = "plain"),
    axis.title.x = element_text(size = 16, family = "Times", face = "italic", hjust = 1),
    axis.title.y = element_text(size = 16, family = "Times", face = "italic", angle = 0)
  )

plot(base_plot)
ggsave("base_plot.png")

base_plot +
  transition_reveal(x)
anim_save("animation_plot.gif")

Output: The two plots are shown below.