Orientation-aware key glyphs

[mjskay]

This PR closes #4732 by providing orientation-aware key glyphs for geom_boxplot(), geom_crossbar(), geom_pointrange(), and geom_linerange().

For example, before these changes:

library(ggplot2)
library(patchwork)

df <- data.frame(
  middle = 1:2,
  lower = 0:1,
  upper = 2:3,
  min = -1:0,
  max = 3:4,
  group = c("a","b")
)

p <- ggplot(df, aes(
  x = middle,
  y = group,
  xmiddle = middle,
  xlower = lower,
  xupper = upper,
  xmin = min,
  xmax = max,
  color = group
))

(p + geom_boxplot(stat = "identity")) +
  (p + geom_crossbar()) +
  (p + geom_linerange()) +
  (p + geom_pointrange())

^{Created on 2022-03-15 by the reprex package (v2.0.1)}

After these changes:

Specifically, this PR:

• modifies draw_key_boxplot(), draw_key_crossbar(), and draw_key_pointrange() to be orientation-aware
• provides a new key glyph, draw_key_linerange(), which is an orientation-aware version of draw_key_path() (because draw_key_path() is used by other geoms whose legends should not be orientation-aware, I could not simply make draw_key_path() orientation aware).
• changes the key glyph of geom_linerange() to be draw_key_linerange() instead of draw_key_path().
• adds tests for horizontal and vertical legends. This might be too many graphical tests; I'm happy to reduce them --- perhaps by removing the vertical tests (the old behavior) and just leaving the horizontal ones. I have condensed the visual tests down to just two for horizontal legends; I'm not sure there's a clear way to reduce them to fewer.

This PR does not add orientation-aware key glyphs to geom_errorbar() or geom_errobarh(), as these geoms consist of both horizontal and vertical lines, so it seemed reasonable to leave them as is. In fact, geom_errorbarh() already has the same key glyph as geom_errorbar() rather than one of the opposite orientation, corroborating my sense that the legends for these two geoms do not need to be orientation-aware.

[hadley]

Looks great, thanks @mjskay