Viewports

• ((draw-viewport viewport) color)
  draw-viewport takes a viewport descriptor. It returns a function that colors the entire contents of viewport. The optional color argument defaults to black.
• ((clear-viewport viewport))
  clear-viewport takes a viewport descriptor. It returns a function that whitens the entire contents of viewport.
• ((flip-viewport viewport))
  flip-viewport takes a viewport descriptor. It returns a function that flips the contents of viewport.
• ((copy-viewport source-viewport destination-viewport))
  copy-viewport takes two viewport descriptors. It copies the source-viewport into the destination-viewport.