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.
This is the equivalent snapshot function to pango_cairo_show_layout().
Not to be confused with gtk_snapshot_render_layout(), which is the
equivalent to gtk_render_layout().
Copy the PangoCairoRenderer into GTK+, rename it to GskPangoRenderer,
and strip it down far enough to build without private pango apis.
This means we currently don't support hexboxes or shapes.
Currently, this lives in gtk, but it might be nicer to put it
in gsk eventually.
