Major issue: The medium and port layers should be merged. There is no gain in performance
to be had by separating them, since transformations need to be applied to the coordinates at
both layers anyway, and if the two layers are coalesced, there is better opportunity for caching
transformations. | SWM, York

12.7.1 General Behavior of Drawing Functions

Using draw-line* as an example, calling any of the drawing functions specified above results in
the following series of function calls on a non-output recording sheet:

• A program calls draw-line* on arguments sheet, x1, y1, x2, and y2, and perhaps some
  drawing options.
  
  
• draw-line* merges the supplied drawing options into the sheet's medium, and then calls
  medium-draw-line* on the sheet. (Note that a compiler macro could detect the case where
  there are no drawing options or constant drawing options, and do this at compile time.)
  
  
• medium-draw-line* on the sheet trampolines to the medium's medium-draw-line* method.
  
  
• medium-draw-line* performs the necessary user transformations by applying the medium
  transformation to x1, y1, x2, and y2, and to the clipping region. Then medium-draw-line*
  calls port-draw-line* to do the actual drawing.
  

Note: CLIM implementations should provide "trampoline" methods on sheets that support
the standard output protocol that simply call the same generic function on the medium. Sheets
that support output recording will require extra mechanism before delegating to the medium in
order to implement such functionality as creating output records and handling scrolling.

=> medium-draw-point* medium x y [Generic Function]
Draws a point on the medium medium. The point is drawn at (x;y), transformed by the medium's
current transformation. medium-draw-point* will call port-draw-point* in order to draw the
point.

=> medium-draw-points* medium coord-seq [Generic Function]
Draws a set of points on the medium medium. coord-seq is a sequence of coordinate pairs, which
are real numbers. It is an error if coord-seq does not contain an even number of elements. The
coordinates in coord-seq are transformed by the medium's current transformation. medium-draw-
points* will call port-draw-points* in order to draw the points.

=> medium-draw-line* medium x1 y1 x2 y2 [Generic Function]