Avoid overwriting the reversed_p flags in the glyph matrices.

Avoid overwriting the reversed_p flags in the glyph matrices.

 Avoid overwriting the reversed_p flags in the glyph matrices when
 init_iterator or start_display are called.

 src/xdisp.c (init_iterator): Don't initialize the reversed_p flag of
 the glyph row here.
 (tool_bar_height, redisplay_tool_bar, try_window)
 (try_window_reusing_current_matrix, try_window_id)
 (get_overlay_arrow_glyph_row, display_menu_bar): Initialize the
 reversed_p flag of the iterator's glyph row where the glyph row
 is going to be redrawn.