output: add output_mode enum

This enum provides two means of behaviour for multi-output setups:
extend the display across all outputs, or only use the last one. The
former is the current (and default) behaviour; the latter will be added
in the next commit.

cage.c

@@ -270,6 +270,8 @@ main(int argc, char *argv[])
 		return 1;
 	}
 
+	server.output_mode = CAGE_MULTI_OUTPUT_MODE_EXTEND;
+
 #ifdef DEBUG
 	wlr_log_init(WLR_DEBUG, NULL);
 #else