Empty the bidi cache "stack" when it->sp is zeroed.

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".