Fix flickering of wxStaticBox background in wxMSW.

Fix flickering of wxStaticBox background in wxMSW.

First of all, don't erase background in WM_ERASEBKGND at all if we erase it
anyhow in WM_PAINT, this is totally useless and is what wxBG_STYLE_PAINT is for.

Second, clip out not only the siblings of the static box but also its children
when erasing the background to avoid painting over the controls created as the
box children, which is the preferred way to create them now.

Closes #15150.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74811 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775