Make sprite picking opt-in (#17225)

# Objective

Fixes #16903.

## Solution

- Make sprite picking opt-in by requiring a new `SpritePickingCamera`
component for cameras and usage of a new `Pickable` component for
entities.
- Update the `sprite_picking` example to reflect these changes.
- Some reflection cleanup (I hope that's ok).

## Testing

Ran the `sprite_picking` example

## Open Questions

<del>
   <ul>
    <li>Is the name `SpritePickable` appropriate?</li>
    <li>Should `SpritePickable` be in `bevy_sprite::prelude?</li>
  </ul> 
</del>

## Migration Guide

The sprite picking backend is now strictly opt-in using the
`SpritePickingCamera` and `Pickable` components. You should add the
`Pickable` component any entities that you want sprite picking to be
enabled for, and mark their respective cameras with
`SpritePickingCamera`.

crates/bevy_picking/src/lib.rs

@@ -188,9 +188,12 @@ pub mod prelude {
     };
 }
 
-/// An optional component that overrides default picking behavior for an entity, allowing you to
-/// make an entity non-hoverable, or allow items below it to be hovered. See the documentation on
-/// the fields for more details.
+/// An optional component that marks an entity as usable by a backend, and overrides default
+/// picking behavior for an entity.
+///
+/// This allows you to make an entity non-hoverable, or allow items below it to be hovered.
+///
+/// See the documentation on the fields for more details.
 #[derive(Component, Debug, Clone, Reflect, PartialEq, Eq)]
 #[reflect(Component, Default, Debug, PartialEq)]
 pub struct PickingBehavior {

crates/bevy_sprite/src/picking_backend.rs

@@ -15,8 +15,14 @@ use bevy_render::prelude::*;
 use bevy_transform::prelude::*;
 use bevy_window::PrimaryWindow;
 
+/// A component that marks cameras that should be used in the [`SpritePickingPlugin`].
+#[derive(Debug, Clone, Default, Component, Reflect)]
+#[reflect(Debug, Default, Component)]
+pub struct SpritePickingCamera;
+
 /// How should the [`SpritePickingPlugin`] handle picking and how should it handle transparent pixels
 #[derive(Debug, Clone, Copy, Reflect)]
+#[reflect(Debug)]
 pub enum SpritePickingMode {
     /// Even if a sprite is picked on a transparent pixel, it should still count within the backend.
     /// Only consider the rect of a given sprite.
@@ -30,6 +36,12 @@ pub enum SpritePickingMode {
 #[derive(Resource, Reflect)]
 #[reflect(Resource, Default)]
 pub struct SpritePickingSettings {
+    /// When set to `true` sprite picking will only consider cameras marked with
+    /// [`SpritePickingCamera`] and entities marked with [`PickingBehavior`]. `false` by default.
+    ///
+    /// This setting is provided to give you fine-grained control over which cameras and entities
+    /// should be used by the sprite picking backend at runtime.
+    pub require_markers: bool,
     /// Should the backend count transparent pixels as part of the sprite for picking purposes or should it use the bounding box of the sprite alone.
     ///
     /// Defaults to an inclusive alpha threshold of 0.1
@@ -39,6 +51,7 @@ pub struct SpritePickingSettings {
 impl Default for SpritePickingSettings {
     fn default() -> Self {
         Self {
+            require_markers: false,
             picking_mode: SpritePickingMode::AlphaThreshold(0.1),
         }
     }
@@ -50,13 +63,24 @@ pub struct SpritePickingPlugin;
 impl Plugin for SpritePickingPlugin {
     fn build(&self, app: &mut App) {
         app.init_resource::<SpritePickingSettings>()
+            .register_type::<(
+                SpritePickingCamera,
+                SpritePickingMode,
+                SpritePickingSettings,
+            )>()
             .add_systems(PreUpdate, sprite_picking.in_set(PickSet::Backend));
     }
 }
 
 fn sprite_picking(
     pointers: Query<(&PointerId, &PointerLocation)>,
-    cameras: Query<(Entity, &Camera, &GlobalTransform, &Projection)>,
+    cameras: Query<(
+        Entity,
+        &Camera,
+        &GlobalTransform,
+        &Projection,
+        Has<SpritePickingCamera>,
+    )>,
     primary_window: Query<Entity, With<PrimaryWindow>>,
     images: Res<Assets<Image>>,
     texture_atlas_layout: Res<Assets<TextureAtlasLayout>>,
@@ -73,7 +97,8 @@ fn sprite_picking(
     let mut sorted_sprites: Vec<_> = sprite_query
         .iter()
         .filter_map(|(entity, sprite, transform, picking_behavior, vis)| {
-            if !transform.affine().is_nan() && vis.get() {
+            let marker_requirement = !settings.require_markers || picking_behavior.is_some();
+            if !transform.affine().is_nan() && vis.get() && marker_requirement {
                 Some((entity, sprite, transform, picking_behavior))
             } else {
                 None
@@ -92,11 +117,14 @@ fn sprite_picking(
         pointer_location.location().map(|loc| (pointer, loc))
     }) {
         let mut blocked = false;
-        let Some((cam_entity, camera, cam_transform, Projection::Orthographic(cam_ortho))) =
+        let Some((cam_entity, camera, cam_transform, Projection::Orthographic(cam_ortho), _)) =
             cameras
                 .iter()
-                .filter(|(_, camera, _, _)| camera.is_active)
-                .find(|(_, camera, _, _)| {
+                .filter(|(_, camera, _, _, cam_can_pick)| {
+                    let marker_requirement = !settings.require_markers || *cam_can_pick;
+                    camera.is_active && marker_requirement
+                })
+                .find(|(_, camera, _, _, _)| {
                     camera
                         .target
                         .normalize(primary_window)

examples/picking/sprite_picking.rs

@@ -55,7 +55,7 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
                 let i = (anchor_index % 3) as f32;
                 let j = (anchor_index / 3) as f32;
 
-                // spawn black square behind sprite to show anchor point
+                // Spawn black square behind sprite to show anchor point
                 commands
                     .spawn((
                         Sprite::from_color(Color::BLACK, sprite_size),