* gtk/gtktextlayout.c (set_para_values): Propagate
pg_bg_color to the display struct.
(gtk_text_layout_free_line_display): Free it here.
* gtk/gtktextdisplay.c (render_para): If pg_bg_color
is set, draw a rectangle in that color behind
the paragraph.
* gtk/gtktexttag.c (gtk_text_tag_class_init): Add
paragraph-background, paragraph-background-gdk and
paragraph-background-set properties.
(set_pg_bg_color): A setter for paragraph-background.