tdf#112845 rtl dropdowns in wrong position

for rtl we mirror the position of popdowns, so we need
to know their initial desired position before they have actually
appeared yet and have a valid maGeometry.nX, so fill in the
hoped-for position as the current position

Change-Id: Ic22dc3e5d9922f5d48bbbdaad53caabff57188d8
Reviewed-on: https://gerrit.libreoffice.org/43459Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

vcl/unx/gtk3/gtk3gtkframe.cxx

@@ -1621,6 +1621,9 @@ void GtkSalFrame::SetPosSize( long nX, long nY, long nWidth, long nHeight, sal_u
             nY += m_pParent->maGeometry.nY;
         }
 
+        maGeometry.nX = nX;
+        maGeometry.nY = nY;
+
         m_bDefaultPos = false;
 
         moveWindow(nX, nY);