Break dependency from fj-window to fj-host

[hannobraun]

fj-window is the Fornjot library that uses winit to provide an event loop and pull much of the Fornjot ecosystem together. It is the main dependency on fj-app. fj-host is the library that loads models and is responsible for their interaction.

It is totally reasonable to want to build an app with fj-window, but at the same time not wanting to be bound to fj-host, for example to experiment with different approaches to loading models, or with supporting different programming languages for models.

Currently, this is not possible, as fj-window has a hard dependency on fj-host. I think it would be better to replace the dependency with a generalized interface, that can be used for alternative implementations. I haven't looked into the details of what would required for that yet, but I think it would boil down to passing the interface for fj-host into fj-window's run method.

I can see two scenarios for implementing this interface:

• Use something rather simple, like a channel from crossbeam-channel. I suspect that this might be a bit too simplistic.
• Define a custom trait in fj-interop and implement that trait in fj-host.

It would also be possible to achieve the same goal in a different way, by instead creating traits inside fj-host to make loading/evaluating/updating models pluggable behaviors.

[hannobraun]

This issue is no longer applicable. The affected code has been removed. See A New Direction for context.