Problem: `tui->space_buf` may be smaller than the width of the TUI or widest
grid, causing an overflow when calling `tui_grid_clear` if `print_spaces` is
called from `clear_region` (clears the TUI's screen since #23428).
Solution: resize `space_buf` to be wide enough to fit the TUI or widest grid.
Didn't bother shrinking the allocation if the max widths decrease.