Add optional alpha / transparency passthrough for UI picking backend

[mgi388]

What problem does this solve or what need does it fill?

Optional transparency passthrough was added to the sprite picking backend in #16388.

What solution would you like?

• I'd like to have the same capability for the UI picking backend. I have some decorative UI panels and I'd like avoid picking triggering when hovering their transparent areas.
• This was also hinted at in Don't count picking hits on button rounded corners #14941 (comment) but perhaps we forgot to add a tracking issue.
• As a bonus: Pending blessing, I'd consider the implementer doing a per-entity solution from day 1, so that you can mark some entities as using the alpha threshold, while others just inherit the global resource. If I had to guess, it doesn't seem much work adding that at the same time, but time will tell.
• Implementation to be verified but maybe spawning an entity with per-entity picking settings would look like this:

commands.spawn((
    Node {
        // ...
        ..Default::default()
    },
    ImageNode {
        // ...
        ..Default::default()
    },
    ImagePickingSettings { picking_mode: ImagePickingMode::BoundingBox },
));

• Noting that, for the sprite picking backend, SpritePickingSettings defaults to { picking_mode: SpritePickingMode::AlphaThreshold(0.1) } and so the example here is to illustrate that this entity is overriding the OOTB alpha-threshold check to instead use a bounding-box check.

What alternative(s) have you considered?

If you just have rounded buttons maybe #14957 already works for you, but it doesn't in my case.