spiral as drawable object
refactored shadows

fix indentation
tree is just only a collection of spirals and screen render a generic Drawable Object
tree has not more it's and moved spiral lineSegment concat in #lineSegments(offset) of tree
now a drawable object compute children's lineSegment and concat with its
own

Now a tree is just an object with (for now) 4 spirals.
in screen.coffee tree as simple drawable object