buffer->top_line could end up being NULL if we blindly TAILQ_PREV...

with the new implementation of redraw_window, we need to scroll a line more to "push" for a new page scroll.

redraw_window in ui.c already compute the curs_y by itself, so don't update it from here. Also, introduce forward_line: moves the cursor up/down n lines. It's amazing sometimes how a good interface can simplify a large chunk of code!

instead of using the line_off + vline_nth, cache the current top line.