Added support for customizable imaging of characters. The per-buffer

Added support for customizable imaging of characters.  The per-buffer
variable CHAR-IMAGE-STRINGS holds a vector of 256 string which determine
how characters are displayed.  Using an approriate vector will allow
the use of fonts with extra characters, `hex' displays, compound characters
for displaying ISO Latin text in plain ASCII and lots of other tricks.

Main components of this update:
 . image.scm to pass in the char-image-strings
 . bufwin.scm to cache the buffer-local variable
 . callers of the imaging routines now get the caches variable
   and pass it to the imaging routines.
 . fixed a few buglets in the caching of buffer-local variables
 . changed the criteria for calling the direct-output operations so that
   they are only used for characters or strings which will be displayed
   as themselves (comred.scm, winout.scm).