code: public Matrix4f set3x3(Matrix4f mat) {
          MemUtil.INSTANCE.copy3x3(mat, this);
          properties &= mat.properties & ~(PROPERTY_PERSPECTIVE);
          return this;
      }
docs: Set the upper left 3x3 submatrix of this {@link Matrix4f} to that of the given {@link Matrix4f}
  and don't change the other elements.
  
  @param mat
  the {@link Matrix4f}
  @return this