Fix posn-at-point in Flycheck buffers

Fix posn-at-point in Flycheck buffers

* src/dispnew.c (buffer_posn_from_coords): Improve commentary.

* src/xdisp.c (move_it_in_display_line_to): Don't exit the loop
under truncate-lines if the glyph at TO_CHARPOS was not yet
produced.  This avoids bailing out too early when we are at
TO_CHARPOS, but didn't yet produce glyphs for that buffer
position, because the last call to PRODUCE_GLYPHS at this position
was for an object other than the buffer.  For further details, see
http://lists.gnu.org/archive/html/emacs-devel/2018-01/msg00537.html.