packet-vnc.c: Improve dissection

- Properly dissect multiple VNC PDUs in one (or more) TCP segment(s).
- Dissect additional message types ('Fence' and 'Enable Continuous Updates').
- Handle "num_rects" field = 0xFFFF (TightVNC).
- Add some more info as to sources of information about the VNC protocol.
- Add an XXX note as to the (incorrect) reassembly method being used.
- Add some notes as to possible ToDo's.

Backported to master-1.12 from master

Change-Id: Id4942c50b3d1373bd2e72c0131614835dc39ba90
Reviewed-on: https://code.wireshark.org/review/2834
Reviewed-by: Bill Meier <wmeier@newsguy.com>
Reviewed-on: https://code.wireshark.org/review/3004