UI z-ordering fix

[ickshonpe]

Objective

During the migration to required components a lot of things were changed around and somehow the draw order for some UI elements ended up depending on the system ordering in RenderSystems::Queue, which can sometimes result in the elements being drawn in the wrong order.

Fixes #19674

Solution

• Added some more stack_z_offsets constants and used them to enforce an explicit ordering.
• Removed the stack_index: u32 field from ExtractedUiNodes and replaced it with a z_order: f32 field.

These changes should fix all the ordering problems.

Testing

I added a nine-patched bordered node with a navy background color to the slice section of the testbed_ui example.
The border should always be drawn above the background color.