facet_grid(space = "free") can work with coord_fixed()

[teunbrand]

This PR aims to fix #4584.

Briefly, we set the ratio of a panel when space = "free" and using coord_fixed().

Previously, in #4432 it was prohibited to set an aspect ratio in the theme and using free space.
However, there is a subtlety here. The theme aspect ratio controls the width/height of panels directly and you cannot reasonably set this ratio for multiple panels of disparate sizes. So theme aspect ratios still aren't allowed. The coord aspect ratio controls the data proportion that is consistent with free facet space, so this should be allowed.

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

df <- data.frame(
  x = c("length a", "length b", "area", "mass", "density", "height"),
  y = c("a", "b", "c", "c", "b", "a"),
  f = c("A", LETTERS[1:5])
)

ggplot(df, aes(x, y)) +
  geom_tile() +
  facet_grid(~f, scales = "free", space = "free") +
  coord_fixed(1)

^{Created on 2024-07-05 with reprex v2.1.0}