Increase range of ComparingUpdateTracker statistics
32 bits are not enough to keep track of all the pixels that might
flow through the system. Expand things to 64 bits, which should cover
all reasonable uses.
Make the comparing update tracker a bit more flexible. It can now be in an
"auto" state where it will be enabled until we deem that the client is better
of without it (currently triggered by explicitly stating a low compression
level).
git-svn-id: svn://svn.code.sf.net/p/tigervnc/code/trunk@4809 3789f03b-4d11-0410-bbf8-ca57d06f2519