XCB: Drag and drop shaped pixmap window doesn't respect HiDPI scaling

Drag and drop shaped pixmap window doesn't respect HiDPI scaling, so the the pixmap is e.g. small and clipped.

Several aspects here:

• QShapedPixmapWindow is clipped if the DnD pixmap has a mask since QWindow::setMask() does not scale the mask (as opposed to QWidget::setMask()). This is the case for item view DnD (QTBUG-46068) but not for tests/manual/highdpi. To reproduce: Use example from QTBUG-46068 with scaling.
• QShapedPixmapWindow has the wrong size if the pixmap has a DPR != 1. To reproduce: Apply https://codereview.qt-project.org/#/c/148747/ , drag text with scaling, note white border around DnD window.