Track the X Shape input region and use it for picking

We now track whether a window has an input shape specified via the X
Shape extension. Intersecting that with the bounding shape (as required
by the X Shape extension) we use the resulting rectangles to paint
window silhouettes when picking. As well as improving the correctness of
picking this should also be much more efficient because typically when
only picking solid rectangles then the need to actually render and issue
a read_pixels request can be optimized away and instead the picking is
done on the cpu.