Design expansion of the declarative UI to support HUD

[jlisee]

It's common in XVIZ clients to want to have specialized always visible UI elements that provide the user situational awareness about what's happening in the scene they are viewing.

We want to:

• Adapt the existing declarative UI system to describe these with minimal changes
• Allow applications to still have a custom HUD
• Support multiple HUD elements at once
• Allow for simple and complex HUDs
• Be drawable in simpler XVIZ clients like a 2D video renderer or image renderer

An example of a HUD are the UI elements on the upper right portion of screen here (source uber/streetscape.gl#200, shoutout to @Pessimistress for the great work here):

Note that this is distinct from the normal panel abstraction in declarative UI where you have sections of the UI that come and go based on purpose.

@twojtasz

[twojtasz]

Another thread came up which raises how declarative UI should work with the ID of primitive. One example would be to have the decl UI instance associated with streams, but on a per ID basis.

Another question is how the HUD would work with multiple poses. There is a disconnect in the way we track objects across streams (ie with the id field) and how we might do that with streams in the poses top-level set. This could be ignored/separated out for, but worth at least mentioning here

[jlisee]

Ok so I think what is missing here is that almost all the streams can actually contain data from multiple objects, but must declarative UI elements ignore that. For many widgets they can just draw multiple things, but for these HUD controls they can only show one number, so you would need a way to specify which ID to use.

This a great find so I created #259 to handle this.