Cut fold and sign columns at end of buffer

win_draw_end() is used to clear the end of the screen in certain cases,
e.g. very long, wrapped lines or the actual end of the buffer.

Fold and sign columns get continued although that's not useful at all
for the case of the end of buffer.

Therefore win_draw_end() gets split into:

    draw_end_of_screen()
    draw_end_of_buffer()

draw_end_of_screen() does does the same win_draw_end() did before and
draw_end_of_buffer() will care about the actual lines after the last
line in the buffer only.