Invalidate VNC framebuffer on every resize.

On a resize, vncviewer keeps the image in the upper-left part of the screen
unmodified and fills the right end of the lines with black colour.  vnc.c       keeps the first n bytes of the framebuffer unmodified instead - meaning that
the client's image doesn't match the image in framebuffer and checking
for dirty rows gives wrong results.  We can either invalidate the whole
buffer or implement the same transformation as the client.


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@4569 c046a42c-6fe2-441c-8c8c-71466251a162
