Initialize scrollbar positions in wxGTK correctly.

Initialize scrollbar positions in wxGTK correctly.

The scrollbar positions stored in wxWindow::m_scrollPos were not initially
correct in wxGTK because wxScrollHelper::SetScrollbars() didn't update them
and only set the values of the underlying GtkAdjustments themselves. This
resulted in filtering out of the first scroll event as the code (wrongly)
believed that the scrollbar position hadn't changed.

Fix this by setting m_scrollPos to the real scrollbar positions.

Closes #12468.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66142 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775