Investigate how Context could be broken up

[17cupsofcoffee]

Summary:
Tetra using a giant Context struct for all of its internal state is a bit of a pain - it makes it impossible for the internals of the renderer to use the public graphics API (because you can't pass ctx and something stored in ctx into the same method), and it's tricky to break it up into something that people can use in a custom game loop.

For me, the main constraints on making a change like this would be:

• No implicit state - I do not want to simply hide Context away in a static and pretend it's not there.
• No added API complexity - functions should not take any more arguments than they already do, just more specific ones
• No added boilerplate - users shouldn't have to write their own game loop from scratch/initialize individual chunks of the engine unless they specifically want/need to do so

I'm not sure if there's an API that meets those criteria! But it's worth thinking about.

[17cupsofcoffee]

Been thinking about this some more, and I think I can articulate what's bugged me about my previous attempts at splitting up Context:

Say we broke up Context into Graphics/Window/Input structs. Cool, that feels more decoupled. But big chunks of a game are going to need access to more than one of those - heck, update could need all of them! So either Tetra or the end user has to pack them up in a struct to pass them around (in which case we're back to having a big Context object) or people have to pass down three arguments where they used to only pass one. I don't think that actually improves anything for end users; in fact if anything it makes their life harder...

[rghartmann]

Have you thought about attribute-based access? If an update() call needs the delta time, the user can request it by:

fn update(#[time] delta: Duration) -> tetra::Result {}

Otherwise, the function would have no parameters at all, and be called as is by Tetra as part of the loop.

I'm not sure if it'd complicate working with the State trait in any way, though (somewhat new to Rust myself). But I am taking inspiration from how Legion handles systems.