Improve accuracy of scrolling commands

Improve accuracy of scrolling commands

* src/xdisp.c (move_it_vertically_backward): Don't rely on
line_bottom_y for accurate calculation of the next screen line's Y
coordinate: it doesn't work when the current screen line was not
yet traversed.  Instead, record the previous Y coordinate and
reseat there if overshoot is detected.
* src/window.c (window_scroll_pixel_based): Calculate the new
window-start point more accurately when screen lines have uneven
height.  (Bug#8355)