stop caching screen_bottom1

stop caching screen_bottom1

I'm not sure this is very useful. I had an initial idea to stop using
screen_bottom1 in final_text_loc_on_screen, by starting from screen_top1
rather than screen_bottom1. But that changes the direction in which we
scan for the text line in situations where there is somehow no text on
screen (something that should never happen but I have zero confidence in
that).

Still, it doesn't seem like a bad thing to drastically reduce the
lifetime of some derived state.

Really what I need to do is throw this whole UX out and allow the cursor
to be on a drawing as a whole. So up arrow or left arrow below a drawing
would focus the whole drawing in a red border, and another up arrow and
left arrow would skip the drawing and continue upward. I think that
change to the UX will eliminate a whole class of special cases in the
code.