Implement Mat3 inversion

Inversion is done by LU-decomposition and further equations solving:
  L * Z = I
  U * X = Z
  L * U * X = I
where:
  L and U are lower and upper triangular matrices got by
  Mat3::decompose(),
  X and I are respective columns of inverse and identity matrices.

Required decompose(), solveL(), solveU() methods are implemented as
well.