vnc: cleanup surface handling, fix screen corruption bug. (Gerd Hoffmann)

This patch killes the old_data hack in the qemu server and replaces
it with a clean separation of the guest-visible display surface and
the vnc server display surface.  Both guest and server surface have
their own dirty bitmap for tracking screen updates.

Workflow is this:

(1) The guest writes to the guest surface.  With shared buffers being
    active the guest writes are directly visible to the vnc server code.
    Note that this may happen in parallel to the vnc server code running
    (today only in xenfb, once we have vcpu threads in qemu also for
    other display adapters).

(2) vnc_update() callback tags the specified area in the guest dirty
    map.

(3) vnc_update_client() will first walk through the guest dirty map.  It
    will compare guest and server surface for all regions tagged dirty
    and in case the screen content really did change the server surface
    and dirty map are updated.
    Note: old code used old_data in a simliar way, so this does *not*
    introduce an extra memcpy.

(4) Then vnc_update_cient() will send the updates to the vnc client
    using the server surface and dirty map.
    Note: old code used the guest-visible surface instead, causing
    screen corruption in case of guest screen updates running in
    parallel.

The separate dirty bitmap also has the nice effect that forced screen
updates can be done cleanly by simply tagging the area in both guest and
server dirty map.  The old, hackish way was memset(old_data, 42, size)
to trick the code checking for screen changes.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6860 c046a42c-6fe2-441c-8c8c-71466251a162
