Fix subtle problem with scroll-down when scroll-margin is nonzero

* src/window.c (window_scroll_pixel_based): Account for
scroll-margin when scrolling down, i.e. moving window-start
towards the beginning of the buffer.  Reported by zhang cc
<ccsmile2008@outlook.com> in
http://lists.gnu.org/archive/html/emacs-devel/2017-12/msg00894.html.

src/window.c

@@ -5397,12 +5397,13 @@ window_scroll_pixel_based (Lisp_Object window, int n, bool whole, bool noerror)
       if (it.what == IT_EOB)
 	partial_p =
 	  it.current_y + it.ascent + it.descent
-	  > it.last_visible_y - WINDOW_HEADER_LINE_HEIGHT (w);
+	  > it.last_visible_y - this_scroll_margin - WINDOW_HEADER_LINE_HEIGHT (w);
       else
 	{
 	  move_it_by_lines (&it, 1);
 	  partial_p =
-	    it.current_y > it.last_visible_y - WINDOW_HEADER_LINE_HEIGHT (w);
+	    it.current_y
+	    > it.last_visible_y - this_scroll_margin - WINDOW_HEADER_LINE_HEIGHT (w);
 	}
 
       if (charpos == PT && !partial_p