bugfix: crash when using mouse wheel

bugfix: crash when using mouse wheel

All the Text functions assume the cursor is always on a text line. I was
violating that invariant.

* When scrolling up, I start the cursor at the top-most line below the
  screen top.

* When scrolling down, I start the cursor at the top-most line below the
  screen bottom.

  I think it would feel slightly more natural for it to be the
  bottom-most line above the screen bottom.

  However, the Text functions maintain an invariant that the bottom-most
  line in a buffer will be text. There's no such invariant for the
  top-most line.