Add relative position reporting to UI picking

[bytemunch]

Objective

Add position reporting to HitData sent from the UI picking backend.

Solution

Add the computed normalized relative cursor position to hit_data alongside the Entity.

The position reported in HitData is normalized relative to the node, with (0.,0.,0.) at the top left and (1., 1., 0.) in the bottom right. Coordinates are relative to the entire node, not just the visible region.

HitData needs a Vec3 so I just extended with 0.0. I considered inserting the depth here but thought it would be redundant.

I also considered putting the screen space position in the normal field of HitData, but that would require renaming of the field or a separate data structure.

Testing

Tested with mouse on X11 with entities that have Node components.

---

Showcase

// Get click position relative to node
fn hit_position(trigger: Trigger<Pointer<Click>>) {
    let hit_pos = trigger.event.hit.position.expect("no position");
    info!("{}", hit_pos);
}

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[ickshonpe]

Works fine, added

                .observe(|trigger: Trigger<Pointer<Click>>| {
                    let hit_pos = trigger.event.hit.position.expect("no position");
                    info!("{}", hit_pos);
                });

to the interactable node in the relative_cursor_position example and the position reported by info! matches the position reported by the example's position text.

I think not repurposing the normal field is a good decision as it's potentially confusing.