Improve Context Traits

[cwfitzgerald]

Full dump of the various proposals for improving the context api cwfitzgerald@f508bbd

Notable points is that there will be a trait for every type with any inherent methods. It will be stored in a thing like this:

enum InterfaceType<Core, WebGPU> {
    Core(Core),
    WebGPU(WebGPU),
}

impl<Core, WebGPU> InterfaceType {
    fn as_core(&self) -> &Core {
        match self {
            InterfaceType::Core(value) => value,
            _ => panic!("AdapterInterfaceType is not core"),
        }
    }

    fn as_webgpu(&self) -> &WebGPU {
        match self {
            InterfaceType::WebGPU(value) => value,
            _ => panic!("AdapterInterfaceType is not webgpu"),
        }
    };
}

// For each type with a trait:
impl Deref for InterfaceType<crate::wgpu_core::Adapter, crate::webgpu::Adapter> {
    type Target = dyn AdapterInterface;

    fn deref(&self) -> &Self::Target {
        match self {
            InterfaceType::Core(value) => value,
            InterfaceType::WebGPU(value) => value,
        }
    }
}

Then do the same trick for the Context

enum HoldMeAContext {
    Core(crate::wgpu_core::CoreContext),
    WebGPU(crate::webgpu::WebGPUContext),
}

impl Deref for HoldMeAContext {
    type Target = dyn Context;

    fn deref(&self) -> &Self::Target {
        match self {
            InterfaceType::Core(value) => value,
            InterfaceType::WebGPU(value) => value,
        }
    }
}

The context trait will do all the as_core/as_webgpu calls inline right at the beginning.