[C] Fix unwanted point moveage in VM

[Stephen J. Turnbull]

Aidan Kehoe writes:

 > It should be possible to avoid this in VM. AIUI it’s a result of
 > doing coding system and base64, quoted-printable decoding on
 > displayed text, which will always cause a redisplay flash if the
 > relevant text is being shown.  (Text is inserted, more text is
 > deleted). It would make more sense to do all the decoding somewhere
 > that's not being redisplayed, and only show the text when the
 > decoding is finished.

I guess you could double buffer, have one buffer for the current view
(or preview) and another where the work is being done.  Is that what
you have in mind?