textview: Add draw_layer vfunc

textview: Add draw_layer vfunc

This allows subclasses to render things below and above the text
in the text view. This allows e.g. GtkSourceView to highlight the
cursor row and to render overlays for colum 80. This used to be done
by rendering before/after chaining up to the parent, but that doesn't
work anymore since the view now renders a background, and due to the
use of the pixel cache.