Gradient tool gizmos for setting radial ellipse's minor and major axis

This involves the Gradient tool drawing an ellipse around the gradient and allowing it to be resized along any of the four edges of the ellipse (at its minor and major axis points).

Start by locating the Gradient tool's drawing and gizmo interaction code, then inspect how ellipse handles are represented and resized. Implement an ellipse around the radial gradient with resize controls at the four minor- and major-axis points, and verify that dragging each control changes the corresponding axis.

Written by the indexing model from the issue text.