Stop debug mode from being used without the new backends

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

src/compton.c

@@ -1712,6 +1712,11 @@ static session_t *session_init(int argc, char **argv, Display *dpy,
 		}
 	}
 
+	if (ps->o.debug_mode && !ps->o.experimental_backends) {
+		log_fatal("Debug mode only works with the experimental backends.");
+		return NULL;
+	}
+
 	ps->atoms = init_atoms(ps->c);
 	ps->atoms_wintypes[WINTYPE_UNKNOWN] = 0;
 #define SET_WM_TYPE_ATOM(x)                                                              \