- Fix issue from PR #10 that broke ability to scroll down when in terminal:
  Resolved by making the line height calculations on integers instead of
  floating-point when in terminal.

- Fix minor issue from PR #10 caused when scrolling with mouse:
  When mouse scrolled the buffer all the way down it made the top space
  unexpectedly jump. This was resolved by lowering topspace command hook
  priorities such that the hooks are run sooner in the list.

- PR #10 introduced a bug where scrolling doesn't working as expected
  when Emacs is run in terminal. The fix for this in the previous commit
  was not sufficient. This commit fixes the source of the problem,
  which was that floats were being passed to
  scroll-up/scroll-down from the added advice functions, but that
  seems to be invalid when in terminal. So now only integers are being passed
  to scroll-up/scroll-down when in terminal.

- The buffer would get scrolled up too far by one line when scrolling
  text up in the following condition: top space was present before
  scrolling but not present after, and the top line in the buffer should
  be at the very top of the window after (but the bug made the second line
  be at the top of the window). This is resolved here.
- The technical reason for the bug was that 0.0 would be passed to `scroll-up`
  after `topspace--filter-args-scroll-up`, assuming that `scroll-up` would not
  scroll in this scenario, however it turns out that if
  `scroll-up`/`scroll-down` are passed a zero-valued float they will
  still scroll by one line.