W: width of image, in pixels
H: height of image, in pixels
F: frame count in animation
g: CanvasRenderingContext2D
f: current frame index

g.sprite(x, y, bytes):

  draw a list of bytes at (x,y).
  results may be imprecise if you have
  altered the projection matrix.

g.xor(flag):

  toggle drawing in xor mode.