Arc Tool behaves strangely when dragging one end to the other and back again


![arc_tool_bug](https://github.com/otfried/ipe/assets/14273595/fbfb79f6-e875-4ac4-969b-b798b45d36a7)

## Steps to Reproduce

1. Create Arc
2. Drag one end to the other and back again while holding the left mouse button
3. Move the center of the arc

## Context

- Ipe 7.2.27
- Ubuntu 22.04