def compute_source_path(source, dir, ext)
      source = rewrite_extension(source, dir, ext) if ext
      File.join(config.assets_dir, dir, source)
    end
def compute_source_path(source, dir, ext)
      source = rewrite_extension(source, dir, ext) if ext
      File.join(config.assets_dir, dir, source)
    end
Return the filesystem path for the source