Work around some bugs.

- Crash in VW_UpdateScreenBlocks. Need to really look at these fns,
  they seem to enjoy reading off the end of blockstarts[].
- Apels did not slide down poles properly.
- On "Exit to Tuberia", the keys were not cleared so if loading was
  quick, you'd still be holding the key and would re-enter the map.
  (Vanilla)