Optimize UI model instance

[MjnMixael]

Addresses an issue that came up while reviewing #7225 (Note that this PR branches from that PR, so that one should be merged first) where we really shouldn't be creating instances for UI models every frame.

Here we use some of the model details to create a cache of instances for the UI. Each frame we check for stale instances and clear them as well as clearing all instances between game states. With this framework, the call sites can lazy load model instances as needed and the engine takes care of keeping things tidy. This is especially useful for the Lua side where any number of models can be rendered and we don't want to have to rely on the script to remember to clean up after itself.

Be aware that the code for using destroyed subsystems as part of the instance key is setting up for supporting #7218 . Additionally the tristatebool return type will allow for that code path to skip some model setup each frame if it returns true.

Also adds model instance support for 3D icons and Fixes #7054

[wookieejedi]

I'll defer to @BMagnu for this one

[BMagnu]

I'm a touch wary of using timestamps instead of number of frames unused, but I guess it's fine. It'll be good enough to prevent frequent reinitialization of the model instance every frame when used through scripting

[MjnMixael]

I'm a touch wary of using timestamps instead of number of frames unused, but I guess it's fine. It'll be good enough to prevent frequent reinitialization of the model instance every frame when used through scripting

Changing this to frames was actually trivial. So there we go.