Gtk.TextView: ensure overlays are rendered above ChildAnchors

TextViewChildren are currently rendered before the ChildAnchors get rendered. Since overlay widgets (added with gtk_text_view_add_overlay) are using the center_child TextViewChild to be rendered, the overlays do not work as one'd imagine an overlay to work, since they appear "behind" any ChildAnchors that might have been added.

This commit fixes this by simply moving the snapshot_text_view_child calls behind the loop responsible for rendering the ChildAnchors.