* dispnew.c: Integer signedness and overflow fixes.

* dispnew.c: Integer signedness and overflow fixes.

Remove unnecessary forward decls, that were a maintenance hassle.
(history_tick): Now uprintmax_t, so it's more likely to avoid overflow.
All uses changed.
(adjust_glyph_matrix, realloc_glyph_pool, adjust_frame_message_buffer)
(scrolling_window): Use ptrdiff_t, not int, for byte count.
(prepare_desired_row, line_draw_cost):
Use int, not unsigned, where either works.
(save_current_matrix, restore_current_matrix):
Use ptrdiff_t, not size_t, where either works.
(init_display): Check for overflow more accurately, and without
relying on undefined behavior.