Circle Tool behaves strangely when control points are on a line


![ellipse_tool_bug](https://github.com/otfried/ipe/assets/14273595/f668aa29-f2a2-49ac-91e9-025ef28f32a9)


## Steps to Reproduce

1. Create Circle
2. Drag both control point onto a line
3. Move first control point

## Context

- Ipe 7.2.27
- Ubuntu 22.04