[StThemeNode] Add basic background-position support

Add basic support for background-position, which only supports absolute
values.

Also don't require an unit to be specified for 0 (because the unit does not
really matter here 0 is 0 regardless of the unit).

https://bugzilla.gnome.org/show_bug.cgi?id=624375

src/st/st-theme-node-private.h

@@ -22,6 +22,10 @@ struct _StThemeNode {
   StGradientType background_gradient_type;
   ClutterColor background_gradient_end;
 
+  int background_position_x;
+  int background_position_y;
+  gboolean background_position_set : 1;
+
   ClutterColor foreground_color;
   ClutterColor border_color[4];
   ClutterColor outline_color;