StThemeNode: Fix the confusion between border and background

In d66e7dd49 I got confused between border_texture and
background_texture. The background_texture was being created as normal
but in the one place that it gets drawn I accidentally made it use the
border_material instead. This patch makes it create a
background_material similar to the border_material and uses it to
paint.

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

@@ -77,6 +77,7 @@ struct _StThemeNode {
   CoglHandle background_shadow_material;
   CoglHandle border_shadow_material;
   CoglHandle background_texture;
+  CoglHandle background_material;
   CoglHandle border_texture;
   CoglHandle border_material;
   CoglHandle corner_material[4];