Issue title: Control aspect ratio (or width & height) of images
 Issue body: It would be great to be able to adjust the aspect ratio or explicitly set the width/height instead of just a size parameter when plotting with `geom_image`.

For instance:
`geom_image(aes(image=image), width = 0.5, height = 1)`
`geom_image(aes(image=image), aspect_ratio = 0.5)`
 Comments: 
 Comment 0: you can use `geom_image(aes(image = image), asp = 0.5)`

 Comment 1: however, when using ggsave(), in the saved plot, the images still distorted (e.g., not as asp = 0.5). is there a solution for this? thanks!
 Comment 2: Is it possible to keep the aspect ratio consistent while plotting with ggimage on a Shiny app, where the size of a graph changes with the user’s screen size? I can use the `asp` argument to make images look good on a certain screen size, but if the user has a different screen, images distort yet again. Maybe it is possible to write a formula that changes `asp` in relation to the current window resolution?
{'source': 'GithubIssues', 'repo_name_with_owner': 'GuangchuangYu/ggimage', 'issue_id': 388962809, 'repo_stars': 146.0}