This add the missing cursor move to the top left in case the upper
window is selected as stated in z-spec 8.7.2.1.1. This will make screen
refreshes work again for Seastalker.

Since the blockbuffer doesn't shrink, the buffer contents must not be
realigned and buffer sizes must not be reduced in case the screen gets
smaller.

Blockbuffer will always work, even before split_window has been invoked.
Made fizmo_new_screen_size adapt blockbuffer screen size. The blockbuffer is
now automatically created upon startup.