render: add a 'clean' bit to each cell; only render cells that aren't clean

This patch takes a bit from the foreground color value in a
cell (todo: split up foreground/background into bitfields with a
separate field for 'foreground/background' has been set), and only
re-renders cells that aren't marked as clean.

Note: we use a 'clean' bit rather than a 'dirty' bit to make it easy
to erase cells - we can (keep doing) do that by simply memsetting a
cell range to 0.

grid.c

@@ -29,6 +29,8 @@ grid_row_alloc(int cols)
 {
     struct row *row = malloc(sizeof(*row));
     row->cells = calloc(cols, sizeof(row->cells[0]));
+    for (size_t c = 0; c < cols; c++)
+        row->cells[c].attrs.clean = 1;
     row->dirty = false;  /* TODO: parameter? */
     return row;
 }