Support for `geom_circle()`

[Henryjean]

Would love a way to fill in a geom_point with an image of my choice.

Effectively, what I'd like to be to do is re-create a chart like this one:

Each point on the chart would correspond to a different image url in my dataframe, which is then used to fill in the circle.

I can kind of do this with geom_polygon_pattern() but it'd be nice if I didn't have to draw the circle first

[coolbutuseless]

Note to self (or future code submitter) - geom_circle from {ggforce} is probably the ideal candidate for adaptation.

https://cran.r-project.org/package=ggforce

[trevorld]

Although we'd still welcome a PR that adds a geom_circle_pattern() to {ggpattern} in the future it may be possible to directly use pattern/gradient fills with ggforce::geom_circle(): https://github.com/thomasp85/ggforce/issues/323