Fix for lock recusion in handle_all_expose()

[mherrb]

XCheckIfEvent() holds the X display lock and the predicate
function it calls is not allowed to call any Xlib function that
would re-enter the lock.
libX11 1.8.1 enables X display locks unconditionnaly (it was only
enabled by XInitThreads() when called explicitely before) and
thus exposes the issue.

So don't process events in the FCheckPeekIfEvent() predicate, but
instead use a separate handler that is called for the returned event
out of the lock.

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