Plugin does not work for any project with more than one camera

[Saratii]

Currently the code will produce strange results if you have multiple cameras (most projects do). There is not a way to specify Which cameras should and should not have clouds as it queries a single and will panic if there are multiple cameras.

Here is a minimal modification to the minimal example that breaks the plugin. It is likely taking into account the second camera in the compute shader because there is no way to exclude it.

//! A minimal example featuring clouds.
use bevy::{prelude::*, render::view::Hdr};
use bevy_volumetric_clouds::CloudsPlugin;

fn main() {
    App::new()
        .add_plugins((DefaultPlugins, CloudsPlugin))
        .add_systems(Startup, setup)
        .run();
}

fn setup(mut commands: Commands) {
    commands.spawn((Camera3d::default(), Hdr));
    commands.spawn((
        Camera3d::default(),
        Transform::from_translation(Vec3::new(0., 150., 0.)),
        Camera {
            order: 1,
            ..default()
        },
    ));
}```

[Saratii]

I created this PR. I am not a rendering dev so this may not be perfect but it maintains original functionality while fixing the bug I described.
#21