Empty the bidi cache "stack" when it->sp is zeroed.
Solves crashes due to overflow of bidi cache stack. Make sure cache is saved and restored around all temporary iterations. src/dispnew.c (buffer_posn_from_coords): Save and restore the bidi cache around display iteration. src/window.c (Fwindow_end, window_scroll_pixel_based) (displayed_window_lines, Frecenter): Save and restore the bidi cache around display iteration. src/bidi.c (bidi_unshelve_cache): Ensure we have enough space before restoring the shelved cache. (bidi_cache_ensure_space): Don't assume the required size is just one BIDI_CACHE_CHUNK away. src/xdisp.c (back_to_previous_visible_line_start, reseat_1) (init_iterator): Empty the bidi cache "stack".
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