Cache obscure flag in screen_write_ctx.

tmux.h

@@ -1055,6 +1055,8 @@ struct screen_write_ctx {
 
 	int				 flags;
 #define SCREEN_WRITE_SYNC 0x1
+#define SCREEN_WRITE_OBSCURED 0x2
+#define SCREEN_WRITE_CHECKED_IF_OBSCURED 0x4
 
 	screen_write_init_ctx_cb	 init_ctx_cb;
 	void				*arg;