Window decorations still rendered when fullscreen set to true

**Describe the bug**
`eframe` renders window decorations when in `fullscreen` is `true`.

Workaround: set `decorations = !fullscreen`.

**To Reproduce**
Steps to reproduce the behavior:
1. Install eframe_template
2. Implement F11 to set `fullscreen` to `true` (see code below)
3. Run app
4. Press F11
5. Observe window decorations rendered where the normal-sized window used to be.

**Expected behavior**
Window decorations do not render.

**Screenshots**


**Desktop (please complete the following information):**
- OS: WSL2 in Windows 11
- Browser: N/A
- Version: N/A

**Smartphone (please complete the following information):**
- Device: N/A
- OS: N/A
- Browser: N/A
- Version: N/A

**Additional context**

Fullscreen implementation:

```
if let Some(new_fullscreen) = ctx.input(|i| {
if i.key_pressed(Key::F11) { Some(!i.viewport().fullscreen.unwrap_or(false)) }
else { None }
}) {
ctx.send_viewport_cmd(egui::ViewportCommand::Fullscreen( new_fullscreen));
}
```

Start by reproducing the issue in eframe_template with the provided F11 handler and Cargo.toml setup, focusing on the ViewportCommand::Fullscreen transition. Compare the fullscreen and normal-sized states on WSL2 in Windows 11; done means fullscreen no longer renders the previous window decorations.

Written by the indexing model from the issue text.