Fix bug #18545 with lack of scrolling a window when point goes out of view.

Fix bug #18545 with lack of scrolling a window when point goes out of view.

 src/xdisp.c (cursor_row_fully_visible_p): Update commentary.
 (redisplay_window): Treat the frame's frozen_window_starts flag
 the same way as the optional_new_start flag for the window: only
 obey it if the glyph row showing point will be fully visible.
 Likewise when the window start is in a continuation line.  If,
 after trying everything under the 'force_start' label, point is
 still not fully visible, give up and scroll the window.  Add
 debugging traces.
 src/window.c (Frecenter): Set the window's redisplay flag.