Violin resolution

[teunbrand]

This PR aims to fix #5396.

Briefly, geom_violin() calls resolution() to determine the default width of a violin.
In resolution(), we're (correctly) making an exception for integers and 'mapped_discrete' vectors to have resolution 1.

However, the 'mapped_discrete' class got dropped from data$x due to a mean() operation.
In this PR, we skip the mean calculation when data$x only has 1 unique value, thereby preserving the 'mapped_discrete' class that gets forwarded to resolution().

Small demo:

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

dplyr::filter(diamonds, cut %in% c("Ideal", "Good")) |>
  ggplot(aes(price, cut)) +
  geom_violin() +
  scale_y_discrete(drop = FALSE)

^{Created on 2023-08-29 with reprex v2.0.2}

[yutannihilation]

Looks good to me.

[teunbrand]

Thanks for the review!