rendernodeparser: Add support for reusing textures

rendernodeparser: Add support for reusing textures

We extend the syntax for textures from just:
  <url>
to
  [<string>] <url>
  <string>
where the first defines a named texture while the second references a
texture.
Or to give an example:
  texture {
    bounds: 0 0 10 10;
    texture: "foo" url("foo.png");
  }
  texture {
    bounds: 20 0 10 10;
    texture: "foo";
  }
This will draw the texture "foo.png" twice, once at (0,0) and once at
(20,0).

The intended use for this is both shortening generated node files as
well as allowing to write tests that reuse textures, in particular when
mixing them in texture and texture-scale nodes.