Notes:

  1st write - X1 position (soft reset  = 0)
  2nd write - X2 position (soft reset = 255)
  3rd write - Y1 position (soft reset = 0)
  4rd write - Y2 position (soft reset = 191)
  Reads do not advance the clip position
  When the clip window is enabled for sprites in "over border" mode,
  the X coords are internally doubled and the clip window origin is
  moved to the sprite origin inside the border.