Bevy ignores explicit canvas size on web/webgpu platform

## Bevy version

0.16.1

## What you did

I specify a specific width/height on an HTML canvas
``````,
I set the canvas id on my primary window:
```rust
app.add_plugins((DefaultPlugins.set(WindowPlugin {
primary_window: Some(Window {
canvas: Some("#bevy-canvas".into()),
..default()
}),
..default()
})
```

## What went wrong

Bevy resized my canvas to 1280x720:

## Additional information

Repository reproducing the issue https://github.com/rectalogic/bevy_canvas_size