[PATCH] Fix some edgecases in SYNC

[Christopher James Halse Rogers]

In recent X servers gnome-screensaver's fade-to-screensaver has lost the
ability to be interrupted.  Tim Taiwanese Liim's analysis on the Fedora bug¹
lead to an edge case in the SYNC extension.

If the WakeupHandler is called exactly on the threshold of a NegativeTransition
trigger then SyncComputeBracketValues can unset the upper bracket bound,
resulting in the idle counter not being updated past the threshold.  Since
a NegativeTransition is only triggered in the transition from strictly greater
than to below the threshold, this results in the NegativeTransition not firing.

Bug references:
[1] Fedora: https://bugzilla.redhat.com/show_bug.cgi?id=612620 about 
Ubuntu: http://bugs.launchpad.net/bugs/595555