(struct row_entry): New structure.

(struct row_entry): New structure.
(row_entry_pool, row_entry_pool_size, row_entry_idx, row_table)
(row_table_size, old_lines, new_lines, old_lines_size)
(new_lines_size, run_pool, runs_size, runs): New variables.
(add_row_entry): New function.
(scrolling_window): Use data structures allocated with xmalloc and
held in global variables, instead of allocing them with alloca and
holding them in local variables.  Use a larger hash table whose
size depends on glyph matrix sizes.  Don't use bzero to clear the
hash table; instead, clear used slots only.