Provide a default filesystem loader for OpenAssetFunc

[mcarpenter622]

It was a bit of a pain trying to figure out how to load an MP3 from the file system. I ended up with something like this
`

type TEST struct {
bytes.Reader
}

func (TEST) Close() error {
return nil
}

l.OpenAssetFunc = func(path string) io.ReadCloser {
	v, _ := os.ReadFile(path)
	var result TEST
	result.Reader = *bytes.NewReader(v)
	return &result
}

`

[quasilyte]

For your example, I think it can be simplified a bit. Unless you really want to eagerly read the file. Since there is caching, either way would work. OpenAssetFunc is called only once per every path.

l.OpenAssetFunc = func(path string) io.ReadCloser {
	// May want to check for an error just in case.
	f, _ := os.Open(path)
	return f
}

For games that have 100% of their assets embedded, it would look like this:

//go:embed all:_data
var gameAssets embed.FS

func OpenAsset(path string) io.ReadCloser {
    f, err := gameAssets.Open("_data/" + path)
    if err != nil {
        panic(err)
    }
    return f
}

For games that may want to have external resources it can be a bit more complicated. I would suggest adding some magic prefix to the resource paths, so OpenAsset may understand that:

func MakeOpenAssetFunc(dataFolder string) func(path string) io.ReadCloser {
	return func(path string) io.ReadCloser {
		if strings.HasPrefix(path, "$") {
			// An external file.
			f, err := os.Open(filepath.Join(dataFolder, path[len("$"):]))
			if err != nil {
				panic(err)
			}
			return f
		}
		// An embedded file.
		f, err := gameAssets.Open("_data/" + path)
		if err != nil {
			panic(err)
		}
		return f
	}
}

OpenAsset = MakeOpenAssetFunc("path/to/data/dir")

By using a closure approach, the open asset func can have some "state" like data dir location, etc. Although it might be a global variable too.

[mcarpenter622]

So I tried that:

l.OpenAssetFunc = func(path string) io.ReadCloser {
	// May want to check for an error just in case.
	f, _ := os.Open(path)
	return f
}

But it doesnt work with an infinite loop mp3. Length in that function returns -1.

func LoopMP3(stream io.ReadSeeker) io.ReadSeeker {
	mp3Stream := stream.(*mp3.Stream)
	fmt.Println(mp3Stream.Length())

	return audio.NewInfiniteLoop(mp3Stream, mp3Stream.Length())
}