Bug fix #153: Possible fix for a rare timing issue

Possible fix for a very rare timing issue in focus detection. Compton
may fail to detect the currently focused window, when a window newly
mapped gets focused, we failed to listen to events and get FocusIn from
it in time, and a series of focus change events before it happens stay
in the event queue and puzzled compton. My choice is to force focus
recheck on all focus-related events. More roundtrips to X, but not
necessarily worse performance, due to the high cost of focus flipping
especially when there's a lot of conditions. Thanks to SlackBox for
reporting.  (#153)