Don't change the current pen and brush in DrawHeaderButtonContents().

Renderer methods shouldn't affect the functions drawing on the DC after it but
the wxRendererGeneric implementation did.

Use wxDC{Pen,Brush}Changer classes instead of calling wxDC::Set{Pen,Brush}()
directly.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64308 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775