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

We should be able to specify which viewport UI should be rendered to in order to facilitate use cases such as split-screen UI:

Source: Mario Kart 8 Deluxe

What solution would you like?

The root node should contain data defining which viewport it should relate to.

• The viewport rect should be the layouting context
• This defines which viewport should render the this UI tree / hierarchy
• There should be some relation between viewports and inputs
• I don't think it makes sense to be able to specify this on a per-node basis

What alternative(s) have you considered?

Ideas are welcome!

Additional context

• Somewhat related to Allow multiple root UI Node #1211
• Also related to the topic of render layers for UI

We should add the four-way split-screen to #1974