Flip geom_curve() direction in some circumstances

[teunbrand]

This PR aims to fix #5069.

Briefly, when reversing or flipping scales, we need to adjust the curves in geom_curve() to be consistent. This PR flips curves by swapping the start- and end coordinates.

The approach harcodes some conditions wherein the swap needs to happen as there isn't a sensible way to derive that from the data itself. The downside of this is that a user can implement a reversing transformation with a different name and we wouldn't detect this.

Also geom_curve() wasn't covered by any tests, so I added some visual ones.

Based on the reprexe in the linked issue; note that the curve consistently curves 'outward' (away from the panel centre).
The missing panel grid is due to #6293.

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

p <- 
  ggplot(mtcars, aes(x = drat, y = mpg)) +
  geom_point() +
  annotate("curve", x = 4.5, xend = 4.5, y = 20, yend = 15, curvature = -1, arrow = arrow())

p

p + coord_flip()

p + coord_flip() + scale_y_reverse()

^{Created on 2025-02-14 with reprex v2.1.1}

[thomasp85]

LGTM, though the flip_curve logic will be a nightmare to maintain :-)

Can't see any other way except for ignoring the issue though