textview: port GtkTextView to GskPangoRenderer

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.