Use a 'damage' list to communicate what needs to be updated

Instead of having each cell in the grid track it's own dirtiness, grid
operations now append "damage" to a list.

This list is consumed every time we render the grid.

This allows us to special case some operations, like erase (and in the
future, scroll).

grid.h

@@ -2,6 +2,9 @@
 
 #include "terminal.h"
 
+void grid_damage_update(struct grid *grid, int start, int length);
+void grid_damage_erase(struct grid *grid, int start, int length);
+
 void grid_erase(struct grid *grid, int start, int end);
 
 void grid_cursor_to(struct grid *grid, int row, int col);