size of element_rect()

Thanks for developing this package! I got this warning running contour() this afternoon:

```
Warning messages:
1: The `size` argument of `element_rect()` is deprecated as of ggplot2 3.4.0.
ℹ Please use the `linewidth` argument instead.
ℹ The deprecated feature was likely used in the BIGL package.
  Please report the issue at <https://github.com/openanalytics/BIGL/issues>.

```