Fix shadow panic when a camera is despawned before extraction

[mgi388]

Objective

• I came across a panic in my game where I was spawning a 3D portrait with its own camera and directional light, and later despawning it. It panicked here:

  bevy/crates/bevy_pbr/src/render/light.rs

  Line 1813 in 81127b3

  .unwrap()

thread 'Compute Task Pool (2)' (54205291) panicked at /me/bevy/crates/bevy_pbr/src/render/light.rs:1508:18:
called `Option::unwrap()` on a `None` value                                                                                                                                                                                                    stack backtrace:
   0:        0x10b3f042c - std[7d7552da0923e8b2]::backtrace_rs::backtrace::libunwind::trace
                               at /rustc/ac68faa20c58cbccd01ee7208bf3b6e93a7d7f96/library/std/src/../../backtrace/src/backtrace/libunwind.rs:117:9
   1:        0x10b3f042c - std[7d7552da0923e8b2]::backtrace_rs::backtrace::trace_unsynchronized::<std[7d7552da0923e8b2]::sys::backtrace::_print_fmt::{closure#1}>       

• At the time I couldn't really work out what the bug was, so I reordered my own code and managed to work around it.

Solution

• After working backwards from the panic at the unwrap() site, I discovered (what I think) is the bug: the for loop over the extracted cascades insertion was incorrectly breaking when it came across an unmapped entity, rather than continuing.
• This meant it dropped entries for other valid cameras.
• Note the fix is a change from else { break; } to else { continue; } which is pointless, hence why I just removed it.

Testing

• See below for a runnable repro for this. It's an example, so reviewers can copy this into your local repo and run the example before and after the fix. The example is actually pretty tiny in the end and so it should be clear it's a valid example.

use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, setup)
        .add_systems(Update, spawn_throwaway_cameras)
        .add_systems(Last, despawn_throwaway_cameras)
        .run();
}

fn setup(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<StandardMaterial>>,
) {
    commands.spawn((
        Mesh3d(meshes.add(Cuboid::new(10.0, 0.1, 10.0))),
        MeshMaterial3d(materials.add(Color::WHITE)),
        Transform::from_xyz(0.0, -0.05, 0.0),
    ));

    commands.spawn((
        Mesh3d(meshes.add(Cuboid::new(1.0, 1.0, 1.0))),
        MeshMaterial3d(materials.add(Color::srgb(0.4, 0.5, 0.9))),
        Transform::from_xyz(0.0, 0.5, 0.0),
    ));

    // Directional light. This is the light whose per-camera cascade map ends up
    // missing the main camera's entry.
    commands.spawn((
        DirectionalLight {
            shadow_maps_enabled: true,
            ..default()
        },
        Transform::from_xyz(4.0, 8.0, 4.0).looking_at(Vec3::ZERO, Vec3::Y),
    ));

    // Main camera.
    commands.spawn((
        Camera3d::default(),
        Transform::from_xyz(-2.5, 4.5, 9.0).looking_at(Vec3::ZERO, Vec3::Y),
    ));
}

#[derive(Component)]
struct Throwaway;

const THROWAWAY_CAMERAS_PER_FRAME: usize = 64;

fn spawn_throwaway_cameras(mut commands: Commands) {
    for _ in 0..THROWAWAY_CAMERAS_PER_FRAME {
        commands.spawn((Camera3d::default(), Throwaway));
    }
}

fn despawn_throwaway_cameras(mut commands: Commands, query: Query<Entity, With<Throwaway>>) {
    for entity in query.iter() {
        commands.entity(entity).despawn();
    }
}

• Note: Affected versions includes at least as far back as Bevy 0.18 (my game's version), so this wasn't a 0.19 regression.
• AFAICT, the bug goes as far back as the original PR that introduced this: https://github.com/bevyengine/bevy/pull/7064/changes#diff-adcfed19bbc11b174d4a90cfbf7e633e04fd539a1a69005236efb85b13919e6fR1226, though I didn't check older versions than 0.18.