Quickly make square or circle annotations

A quick "hack" for generating circular polygons is to use the circular brush size, adjusting its size with [ or ] , clicking once, and pressing enter to complete a circular annotation.

This can be useful to generate polygonal region proposals or weakly supervised labels that polygon-based models like instance segmentation can use to detect an object. For example, circular polygons can be used as stand-ins for keypoints in body or facial landmarks, or to detect corners.