Flushing screen updates after 100 ms turned out to be too aggressive and

Flushing screen updates after 100 ms turned out to be too aggressive and
just gave an annoying window blinds effect that made things difficult to
read. Reduce things to 500 ms, which should be sufficient to tell that the
session isn't completely frozen.

git-svn-id: svn://svn.code.sf.net/p/tigervnc/code/trunk@4495 3789f03b-4d11-0410-bbf8-ca57d06f2519