Fix display of cursor in mouse-highlighted face with ':box'

Fix display of cursor in mouse-highlighted face with ':box'

* src/xdisp.c (erase_phys_cursor, show_mouse_face): Adjust
phys_cursor.x as needed if the cursor is inside mouse-highlight.
(get_cursor_offset_for_mouse_face): New function.
* src/dispnew.c (gui_update_window_end): Set
'mouse_face_overwritten_p' if the cursor is in mouse-face, to
trigger more thorough redisplay of the cursor.
(Bug#50660)