Unable to load assets via URL in Windows

Game uses:

```
url = java.net.URL.new "file://" + ASSETS_DIR + file_name
```

which produces a sensible URL that Windows Explorer is happy with, but the game doesn't find those assets to load.

I was able to change it to:

```
url = java.io.File.new ASSETS_DIR + file_name
```

to get it to _work_, but obviously that isn't going to work in a network-based form.
