Removes redundant fields and arguments around handling of dimensions

Specifically, DisplayObject._x, _y, _width and _height are removed, as they're never actually read, but painstakingly kept in sync with they respective values on _currentTransform.

Also, renderDisplayObject only ever uses the given element's _currentTransform, so there's no need to pass that in explicitly.
Merge pull request #729 from tschneidereit/master

Removes redundant fields and arguments around handling of dimensions