Fix dragging bug in drag_to_scroll example (#21752)

# Objective

Fixes #21675. The issue was that the position the drag started on was
never saved so new drags always forced the grid to its initial position.

The original entity hierarchy consists of a `ScrollableNode`, with a
child node (grid parent), which itself has a bunch of nodes as children
(grid tiles). Only the grid node has a `Pickable` component.

There are a few issues in the original code:

* Since grid tiles have `should_block_lower = false` and `is_hoverable =
true`, they will allow drag events to propagate to the grid parent.
Since the grid parent does not have `Pickable`, it blocks further
propagation but does ding a scroll event. This creates two scroll events
observed by the `ScrollableNode`.
* However, neither of these are the same entity as the `ScrollableNode`,
so both events fail the check originally on line 59 (`drag_start.entity
!= drag_start.original_event_target()`).

## Solution

I tried to fix both issues.

* Add `Pickable` to the grid parent with `should_block_lower = true` and
`is_hoverable = false`. This way it does not ding an event nor does it
propagate further. This ensures there is only one scroll event ever
going through.
* Remove the line 59 entity check as it is no longer necessary.

## Testing

Tested on macOS, seems to work fine.

Author: rudderbucky

examples/ui/drag_to_scroll.rs

@@ -51,27 +51,30 @@ fn setup(mut commands: Commands) {
             },
         )
         .observe(
-            |drag_start: On<Pointer<DragStart>>,
+            |_: On<Pointer<DragStart>>,
              mut scroll_position_query: Query<
                 (&ComputedNode, &mut ScrollStart),
                 With<ScrollableNode>,
             >| {
-                if drag_start.entity != drag_start.original_event_target() {
-                    return;
-                }
                 if let Ok((computed_node, mut start)) = scroll_position_query.single_mut() {
                     start.0 = computed_node.scroll_position * computed_node.inverse_scale_factor;
                 }
             },
         )
         .with_children(|commands| {
             commands
-                .spawn(Node {
-                    display: Display::Grid,
-                    grid_template_rows: RepeatedGridTrack::px(w as i32, 100.),
-                    grid_template_columns: RepeatedGridTrack::px(h as i32, 100.),
-                    ..default()
-                })
+                .spawn((
+                    Node {
+                        display: Display::Grid,
+                        grid_template_rows: RepeatedGridTrack::px(w as i32, 100.),
+                        grid_template_columns: RepeatedGridTrack::px(h as i32, 100.),
+                        ..default()
+                    },
+                    Pickable {
+                        is_hoverable: false,
+                        should_block_lower: true,
+                    }
+                ))
                 .with_children(|commands| {
                     for y in 0..h {
                         for x in 0..w {