Allow assets exported from libraries to be preloaded by the consumer

[ealmloff]

Specific Demand

Libraries can't preload assets directly, but they could export an asset that could be preloaded by the final binary

Implement Suggestion

mod library {
    const ASSET: &Asset = manganis::mg!("image.png");
}

// You could preload the asset defined in the library in the final binary
const ASSET: &Asset = manganis::mg!(ASSET.preload());