Correctly render containers when a split container is focused

include/con.h

@@ -68,6 +68,12 @@ bool con_is_floating(Con *con);
  */
 Con *con_inside_floating(Con *con);
 
+/**
+ * Checks if the given container is inside a focused container.
+ *
+ */
+Con *con_inside_focused(Con *con);
+
 /**
  * Returns the container with the given client window ID or NULL if no such
  * container exists.