fixed two bugs with broken sniksnak graphics in SP engine after warp mode

- the more serious of the two bugs was introduced with commit 727d3b5f
  * lots of non-existing sniksnaks may have been displayed after warp mode
- the second bug existed for a long time (maybe this was never done right)
  * some sniksnaks may have been displayed incompletely after warp mode
- this bug was fixed by forcing a complete playfield redraw after warp mode