Allow justification of legends larger than the available space

[thomasp85]

Fix #3635

This PR aims to address the issue of correctly justifying legends if they are larger than the space available. It does so by setting the viewport size to the size of the legend...

re-rendering of the issue example to show it is working

library(ggplot2)
library(magrittr)

p <- iris %>%
  ggplot(
    aes(x = Sepal.Length, 
        y = Sepal.Width, 
        color = Petal.Length, 
        shape = Species, 
        fill = Petal.Width)
  ) + 
  geom_point() + 
  theme(legend.position = "bottom")

p + theme(legend.justification = "left")

p + theme(legend.justification = "right")

^{Created on 2021-04-20 by the reprex package (v2.0.0)}

(@clauswilke I'm setting you as reviewer as I think you are the other person most familiar with the intricacies of grid. If you don't have time just let me know)

[clauswilke]

I should be able to review this afternoon.

[clauswilke]

Is this meant to be part of the PR?

[thomasp85]

Yeah - I forgot to ask for a news bullet in another PRR I merged and just thought I would fold it in here... If you feel this is too bad we can make a separate commit instead