wip: use a sliding window instead of memmove() to scroll

Instead of memmoving a large amount of data on every scroll, use a
sliding window. That is, each time we scroll, we offset origin.

grid.h

@@ -1,3 +1,7 @@
 #pragma once
 
+#include <stddef.h>
 #include "terminal.h"
+
+struct cell *grid_get_range(struct grid *grid, size_t start, size_t *length);
+void grid_memset(struct grid *grid, size_t start, int c, size_t length);