Transform midpoint argument in relevant scales

[teunbrand]

This PR aims to fix #3198.

Briefly, it uses the scale transformation in mid_rescaler() to transform the midpoint argument. This makes it unnecessary to manually transform that value.

Some examples; standard use of midpoint without a transformation:

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

df <- data.frame(x = c(1, 10, 100))

p <- ggplot(df, aes(x, x, colour = x)) +
  geom_point()
p + scale_colour_gradient2(midpoint = 10)

Midpoint is transformed appropriately:

p + scale_colour_gradient2(midpoint = 10, trans = "log10")

Warning when midpoint is invalid:

p + scale_colour_gradient2(midpoint = 0, trans = "log10")
#> Warning in scale_colour_gradient2(midpoint = 0, trans = "log10"): log-10
#> transformation introduced infinite values in `midpoint`.

^{Created on 2023-11-13 with reprex v2.0.2}

[thomasp85]

LGTM once updated to new trans name