Add convenience function for loading a picture directly from a file

[dusk125]

Not directly, but I can update the atlas to mimic this functionality.

The atlas loads the images as image.Image so it can pack them, then creates the pixel.PictureData from the already loaded images.

[dusk125]

Thanks for the review, converting back to draft as I want to test something else out.

[bhperry]

Alternative proposal from discord discussions:

type ImageLoader interface {
    Image(path string) (image.Image, error)
    PictureData(path string) (*PictureData, error)
}

func NewFileLoader(decoder DecoderFunc) FileLoader {
    if decoder == nil {
        decoder = DefaultImageDecoder
    }
    return FileLoader{decoder: decoder}
}

type FileLoader struct {
    decoder DecoderFunc
}

func (fl FileLoader) Image(path string) (image.Image, error) {
    f, err := os.Open(path)
    if err != nil {
        return nil, err
    }
    defer f.Close()

    return fl.decoder(f)
}

func (fl FileLoader) PictureData(path string) (*PictureData, error) {
    img, err := fl.Image(path)
    if err != nil {
        return nil, err
    }
    return PictureDataFromImage(img), nil
}

func NewEmbedLoader(fs fs.FS, decoder DecoderFunc) EmbedLoader {
    if decoder == nil {
        decoder = DefaultImageDecoder
    }
    return EmbedLoader{fs: fs, decoder: decoder}
}

type EmbedLoader struct {
    fs      fs.FS
    decoder DecoderFunc
}

func (el EmbedLoader) Image(path string) (image.Image, error) {
    f, err := el.fs.Open(path)
    if err != nil {
        return nil, err
    }
    defer f.Close()

    return el.decoder(f)
}

func (el EmbedLoader) PictureData(path string) (*PictureData, error) {
    img, err := el.Image(path)
    if err != nil {
        return nil, err
    }
    return PictureDataFromImage(img), nil
}

Interface style allows pixel to provide a powerful set of loaders that come with self-contained state, while also allowing users to implement their own custom loaders (e.g. MyHttpServerLoader) that will work with the pixel ecosystem.