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.