Properly handle opaque regions for CSD X11 windows

Currently, we don't compute the clip region properly for some client-
side decorated applications, for example gedit, due to mixing several
separate coordinate spaces.

This change ensures that the window pixmap shape and the opaque region
are in the same coordinate space - the window pixmap coordinate.

In order to simplify mapping regions from the window pixmap coordinates
to the global screen coordinates, a new helper method was introduced in
the WindowPixmap class - mapToGlobal().