Add scroll functionality to bevy_picking

[colepoirier]

Objective

bevy_picking currently does not support scroll events.

Solution

This pr adds a new event type for scroll, and updates the default input system for mouse pointers to read and emit this event.

Testing

• Did you test these changes? If so, how?
• Are there any parts that need more testing?
• How can other people (reviewers) test your changes? Is there anything specific they need to know?
• If relevant, what platforms did you test these changes on, and are there any important ones you can't test?

I haven't tested these changes, if the reviewers can advise me how to do so I'd appreciate it!

[ickshonpe]

You need to add

 .add_event::<Pointer<Scroll>>()

to InteractionPlugin's Plugin impl, otherwise you get a panic because it can't find the scroll event resource.

[ickshonpe]

With the event registered, there don't seem to be any other problems.

To test the changes, I removed the update_scroll_position from the testbed_ui example (in examples/testbed/ui.rs) and added an observer to the scrollable node:

// Scrolling list
parent
    .spawn((
        Node {
            flex_direction: FlexDirection::Column,
            align_self: AlignSelf::Stretch,
            height: Val::Percent(50.),
            overflow: Overflow::scroll_y(),
            ..default()
        },
        BackgroundColor(Color::srgb(0.10, 0.10, 0.10)),
    ))
    .observe(|
        trigger: Trigger<Pointer<Scroll>>,
        mut scroll_position_query: Query<&mut ScrollPosition>,
    | {
        if let Ok(mut scroll_position) = scroll_position_query.get_mut(trigger.target) {
            let (dx, dy) = match trigger.unit {
                MouseScrollUnit::Line => (trigger.x * 20., trigger.y * 20.),
                MouseScrollUnit::Pixel => (trigger.x, trigger.y),
            };
    
            scroll_position.offset_x -= dx;
            scroll_position.offset_y -= dy;
        }
    })
    .with_children(|parent| {
        // List items
        for i in 0..25 {
            parent
                .spawn((
                    Text(format!("Item {i}")),
                    TextFont {
                        font: asset_server.load("fonts/FiraSans-Bold.ttf"),
                        ..default()
                    },
                    Label,
                    AccessibilityNode(Accessible::new(Role::ListItem)),
                ))
                .insert(Pickable {
                    should_block_lower: false,
                    ..default()
                });
        }
    });

scrolling working as before.

[colepoirier]

You need to add

 .add_event::<Pointer<Scroll>>()

to InteractionPlugin's Plugin impl, otherwise you get a panic because it can't find the scroll event resource.

Done!

[aevyrie]

Nice work, thank you!