- Fixed mk-viii sound sample loader: missing "/".

- Fixed mk-viii sound sample loader: missing "/".
- Fixed triggering of debug assertion "altitude_callout_voice != NULL" when
  altitude callouts were delayed by higher priority warnings.
- Fixed performance bug: MK_VIII::IOHandler::TerrainClearanceFilter::update
  sucked CPU time (top #12 in profiler), due to complex "moving average"
  calculation on a queue of hundreds/thousands of altitude samples.
  Also, pausing the sim caused this queue to grow excessively (by one sample
  per update loop) => resuming the sim after pausing it for a long time
  resulted in the TerrainClearanceFilter sucking even more CPU time...
  => Number of altitude samples is now limited to 5 samples/sim-time-second
  (still more than enough to calculate a stable average).