SHAPE_POLY_SET: AddHole should return the hole idx

Returning the poly index is not useful as it does not allow the use of
other SHAPE_POLY_SET Hole* functions that expect to get the indices of
holes rather than the outline

libs/kimath/src/geometry/shape_poly_set.cpp

@@ -448,7 +448,7 @@ int SHAPE_POLY_SET::AddHole( const SHAPE_LINE_CHAIN& aHole, int aOutline )
 
     poly.push_back( aHole );
 
-    return poly.size() - 1;
+    return poly.size() - 2;
 }