Mirror H scrollbar visibility guard in Panel::doUpdate reset branch

Robosturm review feedback on PR Robosturm#1959: the V-axis branch was tightened
to reset content X only when the scrollbar is hidden, but the H-axis
branch kept a bare else that snapped Y to 0 every frame the scrollbar
was visible but div<=0. Apply the same !getVisible() guard to H so
both axes behave symmetrically.

Author: TheMasterCreed

objects/base/panel.cpp

@@ -150,7 +150,7 @@ void Panel::doUpdate(const oxygine::UpdateState& us)
             m_HScrollbar->setScrollvalue(value);
         }
     }
-    else
+    else if (!m_HScrollbar->getVisible())
     {
         m_ContentRect->setY(0);
     }