Refactor rt.sceneloader:
- Move the logic from SceneLoadContext.setTo(..) that extracts values into a new method.
- Update that logic to support reading inline values in SDL Tags.
- Make the Value interface methods const @safe/@trusted.
- Make the Deserializable.deserialize(..) method take Value by const reference.
- Update in other places as needed.

Also:
- More work on imageio.bmp.
- Add new texture class: rt.texture.Procedure2.
- Add new scene file to test it: lecture4-proc-texture.sdl and make it the default in data/default_scene.path.
- Fix the getPathToDefaultScene() method to strip the whitespace from path to the default scene.
- Make the Texture.getTexColor(..) method to take normal vector by reference
- Small refactoring on rt.shader.