Fix bug #17905 with display of point in partially visible line at end of window.

Fix bug #17905 with display of point in partially visible line at end of window.

 src/xdisp.c (redisplay_window): If redisplay of a window ends up
 with point in a partially visible line at end of the window, make
 sure the amended position of point actually has smaller Y
 coordinate; if not, give up and scroll the display.
 src/window.c (window_scroll_pixel_based): When point ends up at the
 last fully visible line, don't let move_it_to stop at the left
 edge of the line and dupe us into thinking point is inside the
 scroll margin.