Allow setting actual root path for FileSystemResourcePack

[ghost]

This discussion has reminded me, I'd like to propose an option of explicitly setting a root folder for the FileSystemResourcePack.

For example:

public FileSystemResourcePack(IFileSystem fileSystem, string rootPath)

When resolving asset paths the pack implementation will treat these paths as relative to the given root.

I believe that will give following benefits:

• game developer may make the asset's root on disk variable, depending on some configuration, while keeping asset paths in code simple and constant.
• you may add several separate locations, each of which has same inner directory structure.