Render UI To Texture & Multiple Apps

[Braymatter]

Draft: Render a Kayak UI to a Texture

I've got a use case where I'd like to render multiple Kayak UI Applications and Mount them in world space. As an example having computer kiosks in the world

I was on the fence about making these two separate pull requests but mounting an App in worldspace \ render texture would almost definitely be something done in addition to mounting a HUD to the UI camera as well.

Having Multiple Kayak Apps

This would require making BevyContext a component instead of a Resource and providing a system that will call render on each BevyContext. Will also require updating process_events to consume BevyContext components and likely we'll need to create an IndexResource.

Another wrinkle is handling input on WorldSpace UI's. A basic solution could be to use bevy_mod_picking to raycast on click and create a synthetic click event if the clicked object has a BevyContext component.

We can address keyboard inputs to the last clicked UI in worldspace, or if the last click was not on a worldspace UI we can fallback to the UICamera App. We'll probably want to provide the world space UI's some mechanism for automatically yielding an input if camera is out of a certain radius, or possibly a more generic predicate provided by the user

Rendering Multiple Apps

Our initial thought is to have the BevyApp render to a texture and map provide that texture to the User to place within the world as they wish.

[StarArawn]

This is partially finished with the 0.8 update and I've added a basic example here:
#137

Input handling is difficult and I'm not sure exactly how we want to handle that yet.