Currently shadows disregard the overall opacity, so e.g. setting
an ancestor's opacity does not effect the shadow. Fix this by
deferring the setting of the shadow's color until it is painted.
Also move duplicated drawing code from st_theme_node_paint() into
its own function.
https://bugzilla.gnome.org/show_bug.cgi?id=619083
The (optional) spread radius allows to make the shadow bigger without
enlarging the blur value. Mozilla supports this parameter for the
-moz-box-shadow property.
https://bugzilla.gnome.org/show_bug.cgi?id=613832
The idea behind this move is that we have a lot more control over
rendering if StWidget isn't a big pile of actors, and things are
more efficient.
https://bugzilla.gnome.org/show_bug.cgi?id=607500
