Fix bugs #13623 and 13626 caused by changes in revision 111647.

Fix bugs #13623 and 13626 caused by changes in revision 111647.

 src/xdisp.c (window_buffer_changed): region_showing can be negative,
 which still means region is being displayed.
 (redisplay_internal): Resurrect code that forced redisplay of the
 whole window when showing region and the mark has changed.  Record
 the new mark position to allow redisplay optimizations.
 (display_line): If it->region_beg_charpos is non-zero, set the
 window's region_showing member to -1.
 src/window.h (struct window) <region_showing>: Declare ptrdiff_t,
 not bitfield of 1 bit.