Properly invalidate rendering cache when updating orientation (Thanks hercek)

fixes #1445

include/con.h

@@ -356,3 +356,9 @@ void con_set_urgency(Con *con, bool urgent);
  *
  */
 char *con_get_tree_representation(Con *con);
+
+/**
+ * force parent split containers to be redrawn
+ *
+ */
+void con_force_split_parents_redraw(Con *con);

src/con.c

@@ -20,7 +20,7 @@ static void con_on_remove_child(Con *con);
  * force parent split containers to be redrawn
  *
  */
-static void con_force_split_parents_redraw(Con *con) {
+void con_force_split_parents_redraw(Con *con) {
     Con *parent = con;
 
     while (parent && parent->type != CT_WORKSPACE && parent->type != CT_DOCKAREA) {

src/tree.c

@@ -407,8 +407,7 @@ void tree_split(Con *con, orientation_t orientation) {
     Con *parent = con->parent;
 
     /* Force re-rendering to make the indicator border visible. */
-    FREE(con->deco_render_params);
-    FREE(parent->deco_render_params);
+    con_force_split_parents_redraw(con);
 
     /* if we are in a container whose parent contains only one
      * child (its split functionality is unused so far), we just change the