Finish and debug display of invisible text.

Finish and debug display of invisible text.
 xdisp.c (handle_invisible_prop): If we are `reseat'ed, init the
 paragraph direction and set the `reversed_p' flag in the IT's
 glyph row.  Fix exit conditions of the loop that skips invisible
 text.  Update IT->prev_stop after skipping invisible text.  Check
 for additional overlays at IT->stop_charpos, not at start_pos.

Clean up the mess with setting the glyph row reversed_p flag.
 dispnew.c (prepare_desired_row): Preserve the reversed_p flag.

 bidi.c (bidi_cache_find): Use bidi_copy_it instead of copying
 the whole struct (which includes uninitialized parts).
 (bidi_init_it): Don't initialize bidi_it->paragraph_dir.

 xdisp.c (display_line): Remove misplaced setting of
 row->reversed_p flags.  Copy the reversed_p flag to the next glyph
 row.
 (next_element_from_buffer): Check bidi_it.paragraph_dir rather
 than level_stack[0].level.  Reset the reversed_p flag for non-R2L
 paragraphs.