Working with Other Node Types  Although sprites are the most
important element you use when building a game, Sprite Kit
provides many other node classes. Many of these node classes
provide visual content, similar to the SKSpriteNode class.
Others do not directly draw content of their own, but instead
modify the behavior of their descendants in the node tree. Table
6-1 lists all the node classes provided by Sprite Kit, including
the SKScene and SKSpriteNode classes you are already familiar
with.  Table 6-1  Sprite Kit node classes     The class from
which all node classes are derived. It does not draw anything.
A scene is the root node in the node tree. It handles animation
and action processing.   A node that draws a textured sprite.
A node that renders a text string.   A node that renders a shape
based on a Core Graphics path.   A node that plays video
content.   A node that creates and renders particles.   A node
that crops its child nodes using a mask.   A node that applies a
Core Image filter to its c