Don't reset scroll position to zero when window size changes

Aside from just being the proper thing to do, this fixes scrolling with
GTK3 when not using overlay scrollbars, where size events are somehow being
generated during scrolling.
See #17915

src/html/htmlwin.cpp

@@ -818,7 +818,8 @@ void wxHtmlWindow::CreateLayout()
             (
                 wxHTML_SCROLL_STEP, wxHTML_SCROLL_STEP,
                 ScrollSteps(m_Cell->GetWidth(), clientWidth - vscrollbar),
-                ScrollSteps(m_Cell->GetHeight(), clientHeight)
+                ScrollSteps(m_Cell->GetHeight(), clientHeight),
+                m_xScrollPosition, m_yScrollPosition
             );
         }
     }