Window size use logical size by default + inconsistent unit + `WindowDescriptor.resizable` not working

## Bevy version

db55bf550423e741c3362a2ad33f4fea8bcb318c

## Operating system & version

Linux 5.13.19-2-MANJARO

## What you did

First, I tried to set window size to 900 x 600;

```
use bevy::prelude::*;

fn main() {
let window = WindowDescriptor {
width: 900.0,
height: 600.0,
..Default::default()
};

App::build()
.insert_resource(window)
.add_plugins(DefaultPlugins)
.insert_resource(ClearColor(Color::rgb(0.9, 0.9, 0.9)))
.add_system(bevy::input::system::exit_on_esc_system.system())
.run();
}
```

However, the window was near 1080px tall, not 600.

So I set `scale_factor_override: Some(1.0)`, and the window size was correct.

Then, I tried to make window fixed size with `resizable: false`. That didn't work either.

So I set `window.resize_constraints` to 900 x 600. However, the actual window was now 1.75 times bigger than expected.