`frame.set_window_size(ctx.used_size());` is broken

Manually resizing the OS window makes the height of the window increase too much. It will even overflow the screen height, and can actually grow infinitely.

**To Reproduce**
1. Run any native egui app
2. Resize the window horizontally
3. Press Alt+tab to see the window growing vertically

**Expected behavior**
The height of the window should be conserved when resizing the window horizontally, and should increase slower when resizing the window vertically.

**Screenshots**

**Desktop**
- OS: Ubuntu
- Version: 21.04