Fix two issues with pivot points (#382)

- ResetPivotPoint() now truely resets the pivot point (and not only
invalidating it).
- The copy constructor of lcPiece now also copies the state of the pivot
point. Thus, duplicating pieces also duplicates the pivot points.

common/piece.cpp

@@ -44,7 +44,9 @@ lcPiece::lcPiece(const lcPiece& Other)
 	mStepHide = Other.mStepHide;
 	mGroup = Other.mGroup;
 	mFileLine = -1;
+
 	mPivotMatrix = Other.mPivotMatrix;
+	mState |= ( Other.mState & LC_PIECE_PIVOT_POINT_VALID );
 
 	mPositionKeys = Other.mPositionKeys;
 	mRotationKeys = Other.mRotationKeys;

common/piece.h

@@ -535,6 +535,7 @@ public:
 	void ResetPivotPoint()
 	{
 		mState &= ~LC_PIECE_PIVOT_POINT_VALID;
+		mPivotMatrix = lcMatrix44Identity();
 	}
 
 public: