Reframe swallowed windows if depth doesn't match

X will not allow a window with ParentRelative background to be created
or reparented under a window with mismatching color depth.
Deal with this by destroying the container frame and creating a new one
with the right depth upon swallowing.
Defer destruction of the frame window until after the updated tree has
been rendered to avoid some distracting flickering.

Fixes #3297

include/x.h

@@ -49,6 +49,12 @@ void x_reinit(Con *con);
  */
 void x_con_kill(Con *con);
 
+/*
+ * Completely reinitializes the container's frame, without destroying the old window.
+ *
+ */
+void x_con_reframe(Con *con);
+
 /**
  * Returns true if the client supports the given protocol atom (like WM_DELETE_WINDOW)
  *