update m_visibleRectBeforeGeometryUpdate when the padding
change signal gets emitted: aurorae updates its shadow
and postentially shadow size asynchronously after repainting.
that would make the first addRepaintDuringGeometryUpdates
when the winow is moved *for the first time* have a wrong
m_visibleRectBeforeGeometryUpdate that doesn't take into
account the shadow, leaving some leftover shadow on screen