Add deadly water feature

[lahm86]

Currently death tiles in water rooms are ignored, but in TR2 onwards they will kill Lara (e.g. Xian lava rooms or Diving Area acid pools). We could in theory just add the feature without making it configurable as there is nowhere like this in the original game, but there may well be existing custom levels where it could cause problems. So possibly a simple gameflow setting to control it would be best.