Drag and drop does not take into account window shaping

There is a bug in Qt 4.8.0 that it does not take into account shaping in findRealWindow in qdnd_x11.cpp so in some cases drag and drop does not work.

It can happen that there is a window covering all the screen but it is shaped
to only take part of the screen. If that happens, besides the condition of
QRect(attr.x,attr.y,attr.width,attr.height).contains(pos)
we also need to query the server for its region rectangles and make
sure the cursor is inside one of those rectangles. If that does not happen
we have to return 0 so the hierarchical XQueryTree search continues

There is an approved Merge Request at Qt-project.org about this http://codereview.qt-project.org/#change,15062

If you don't have an account at Qt-project.org you can find the patch at https://pastebin.canonical.com/59391/

We need this for Unity-2d