Set a flag on cells are genuinely empty (cleared and never written to)

and use tty_clear_line (which will choose the best escape sequence) to
clear any batches of cells with that flag when redrawing a line from the
stored screen.

tmux.h

@@ -579,6 +579,7 @@ enum utf8_state {
 #define GRID_FLAG_EXTENDED 0x8
 #define GRID_FLAG_SELECTED 0x10
 #define GRID_FLAG_NOPALETTE 0x20
+#define GRID_FLAG_CLEARED 0x40
 
 /* Grid line flags. */
 #define GRID_LINE_WRAPPED 0x1