culling: Use vertex_count not '4' to determine partial culls

We were mistakenly using the constant 4 to determine the number of
vertices that need to be culled for a paint-volume to be considered
fully culled too. This is only ok for 2d volumes and was resulting in
some 3d volumes being considered culled whenever 4 out of 8 vertices
were culled. This fix is simply to reference the vertex_count variable
instead of assuming 4.

clutter/clutter-paint-volume.c

@@ -963,7 +963,7 @@ _clutter_paint_volume_cull (ClutterPaintVolume *pv,
             out++;
         }
 
-      if (out == 4)
+      if (out == vertex_count)
         return CLUTTER_CULL_RESULT_OUT;
       else if (out != 0)
         partial = TRUE;