Don't flood expose events in the xcb plugin.

Don't flood expose events in the xcb plugin.

The X server sends a series of expose events, where the count member
specifies how many expose events are remaining in the current series. By
merging them into an expose region we can send a single expose event to
the lighthouse interface.

Change-Id: If73c9972fe02c5e4137e8742aaaf5679ccea5a09
Reviewed-on: http://codereview.qt-project.org/5366
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>