Add basic support for zero width joiners, GitHub issues 1605 and 2784.

tmux.h

@@ -866,6 +866,7 @@ struct screen_write_ctx {
 
 	int				 flags;
 #define SCREEN_WRITE_SYNC 0x1
+#define SCREEN_WRITE_ZWJ 0x2
 
 	screen_write_init_ctx_cb	 init_ctx_cb;
 	void				*arg;