[EGLFS] front windows paint window is not update properly

When there is an another window in front of the mainwindow, parentless QWidget or from drag and drop when it is moved it is not updated correctly.
It needs to be forced with update.
It can reproduced with the attached code or with the [http://code.qt.io/cgit/qt/qtbas
e.git/tree/examples/widgets/draganddrop/draggableicons]