gutter: snapshot color node for current line
gutter: snapshot color node for current line

We can avoid a bunch of machinery here by just using a premixed color from
the GtkSourceView when possible. Additionally, we can get closer to
supporting generic coloring like #29 will require.

GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview