Fix clear color in CompositingSpace::Srgb

[beicause]

Objective

Split off from #23803, Fix ClearColor with CompositingSpace::Srgb.

Solution

Convert clear color to Srgb

Testing

Code from #9213 (comment):

//!

use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .insert_resource(ClearColor(Color::srgba(0., 0.5, 0., 1.)))
        .add_systems(Startup, |mut commands: Commands| {
            commands.spawn((Camera2d, CompositingSpace::Srgb));

            commands.spawn((
                Sprite::from_color(Color::WHITE.with_alpha(0.5), Vec2::new(512., 512.)),
                Transform::from_xyz(-256., 0., 0.),
            ));

            commands.spawn((
                Sprite::from_color(Color::BLACK.with_alpha(0.5), Vec2::new(512., 512.)),
                Transform::from_xyz(256., 0., 0.),
            ));
        })
        .run();
}

Before:

After: