title: Draw Textured Polygons in SpriteKit
body: <p>I'm building a top-down scrolling game in SpriteKit and am trying to produce realistic terrain.  So far I've positioned and rotated overlapping sprites to create grassy fields, and now I'd like to add a river:</p>
  
  <p>Positioning sprites doesn't allow enough control over the shape of the river - the edges look rather poor quality.  I'd like to draw a custom polygon with a texture - like a SKShape node with a texture.</p>
  
  <p>Is there any way to perform custom drawing, or create a new SKNode subclass that will allow this?  Seems like there should be some "catch-all" method to access the graphics context during a node's render and do something with it, but I'm not finding it in the docs.</p>