Add debug mode

When enabled, the result will be redirected to a window, and compton
won't take over the screen.

Makes debugging easier.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/compton.h

@@ -59,6 +59,8 @@ void discard_ignore(session_t *ps, unsigned long sequence);
 
 void set_root_flags(session_t *ps, uint64_t flags);
 
+xcb_window_t session_get_target_window(session_t *);
+
 /**
  * Set a <code>switch_t</code> array of all unset wintypes to true.
  */