textview: port GtkTextView to GskPangoRenderer
This removes the use of GtkTextDisplay (a PangoRenderer) to use the GskPangoRender which generates render nodes. Part of this means improving the GskPangoRenderer to support the necessary features for displaying a GtkTextView.
Primarily, this is a merging of GtkTextDisplay features into GskPangoRender. Additionally, GtkTextDisplay was removed to allow for gtk_text_layout_snapshot() to be implemented elsewhere.