[CoglPangoDisplayList] Don't store the base color in the display list

[CoglPangoDisplayList] Don't store the base color in the display list

It should be possible render a single PangoLayout with different
colors without recalculating the layout. This was not working because
the color used at the first edit was being stored in the display
list. This broke changing the opacity on a ClutterText.

Now each node in the display list has a 'color override' flag which
marks whether it should use the base color or not. The base color is
now passed in from _cogl_pango_display_list_render_texture. The alpha
value is always taken from the base color.