This will allow to implement new encodings (tight, zrle, ..)
in a cleaner way. This may hurt performances, because some
functions like vnc_convert_pixel are not static anymore, but
should not be a problem with gcc 4.5 and the new -flto.

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>