Provides a way to add arbitrary graphics to a buffer's display. This method is called just before and just after every painting of the buffer.
The before argument is #t when the method is called just
before a painting the contents of the buffer or #f when it is
called after painting. The left, top, right, and
bottom arguments specify which region of the buffer is being
repainted, in buffer coordinates. To get the coordinates for dc,
offset buffer coordinates by adding (dx, dy).
See section
for information about draw-caret.
See also invalidate-bitmap-cache.