This function returns a data frame specifying the coordinates of the
  window.
  
If x is a binary mask window,
  the result is a data frame with columns x and y
  containing the spatial coordinates of each pixel.
  If drop=TRUE (the default), only pixels inside the window are retained.
  If drop=FALSE, all pixels are retained, and the data frame has
  an extra column inside containing the logical value of each pixel
  (TRUE for pixels inside the window, FALSE for outside).
  
If x is a rectangle or a polygonal window,
  the result is a data frame with columns x and y
  containing the spatial coordinates of the vertices of the
  window. If the boundary consists of several polygons, the data frame
  has additional columns id, identifying which polygon is being
  traced, and sign, indicating whether the polygon is an
  outer or inner boundary (sign=1 and sign=-1 respectively).