Compute jitter width by panel

[teunbrand]

This PR aims to fix #3722.

It ensures that the default, resolution-based jitter width is computed per panel instead of per layer.

Reprex from the issue, notice that the jitter has about equal visual spread across panels:

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

set.seed(123)
df <- data.frame(
  y = rnorm(100, 50, 10),
  x = c(rep(1:5, 10), rep(seq(0, 100, length.out = 5), 10)),
  group = c(rep("a", times = 50), rep("b", times = 50))
)

ggplot(df, aes(x = x, y = y)) +
  geom_point(color = "orange", alpha = 0.7) +
  geom_point(position = "jitter", color = "blue", alpha = 0.7) +
  facet_wrap( ~ group, scales = "free")

^{Created on 2025-02-12 with reprex v2.1.1}

An open question is whether we should do this for position_jitterdodge() too.

[thomasp85]

LGTM