fix vnc screen corruption bugs and viewer exits

fix vnc screen corruption bugs and viewer exits

I have been having problems with display corruption using raw
encoding.

I have also been having issues with the vncviewer client exiting with
"rect too big".

These problems appear when there are rapid changes to the framebuffer
contents. [I've been using "find /" in my tests to generate a lot of text
output.]

The root cause of both problems appears to be the changing of
the underlying framebuffer data during the creation of the update.
[Hextile makes multiple passes over the data and gets badly
confused if the data changes..]

The attached patch pulls the pixel data from the "stable" old_data
buffer rather than directly from the (changing) display surface.

Submitted-by: Andrew Thomas <andrew.thomas@oracle.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>