xcb_io: Allow jumps backwards when widening the request number.

Request numbers are not always seen in the numeric order by widen(),
for example due to Mesa directly calling _XError(). When this happens,
widen() adds 2^32 to the reported widened number, triggering failed
assertions and bad behavior.

With this commit, wrapping of the lower dword is detected in a more
robust way, by requiring that a skip of at least 2^31 is seen.

This fixes issue #152.

Signed-off-by: Giovanni Mascellani <gmascellani@codeweavers.com>