Don't update cells in each block of data read from a pane immediately,

instead track them as change (dirty) and update them once at the end,
saves much time if repeatedly writing the same cell. Also fix comparison
of cells being equal in a few places (memcmp is not enough).

screen.c

@@ -38,6 +38,9 @@ screen_init(struct screen *s, u_int sx, u_int sy, u_int hlimit)
 	s->ccolour = xstrdup("");
 	s->tabs = NULL;
 
+	s->dirty = NULL;
+	s->dirtysize = 0;
+
 	screen_reinit(s);
 }
 
@@ -64,6 +67,7 @@ screen_reinit(struct screen *s)
 void
 screen_free(struct screen *s)
 {
+	free(s->dirty);
 	free(s->tabs);
 	free(s->title);
 	free(s->ccolour);