MapAndreas and FCNPC_SetMinHeightPosCall fix

[IS4Code]

This pull request fixes two (unrelated) things:

• The integration with MapAndreas does not work at all: the original natives use their own instance, but the CServer has its own instance that is never initialized. This simply uses the natives' instance.
• FCNPC_SetMinHeightPosCall causes the new Z coordinate not to be applied when the difference is within the accepted range (otherwise FCNPC_OnChangeHeightPos is called which works fine). This changes that so that the new value is rejected if and only if FCNPC_OnChangeHeightPos returns false.