Improve scheduling on the Mac:

Improve scheduling on the Mac:
1: Only update the screen when there's nothing else to do.  This means that
it's a lot harder for a fast typist to outrun PuTTY.
2: Only sleep for at most 100ms at a time.  This is a kludge to work around
the WakeUpProcess caused by incoming data can happen before the
WaitNextEvent it's meant to interrupt, leading to PuTTY sleeping forever
because it doesn't know there's network data pending.

git-svn-id: svn://svn.tartarus.org/sgt/putty@2901 cda61777-01e9-0310-a592-d414129be87e