math/matrix: Don't use rcp[ps]s

! It's used once per transposed_invert(), so it hurts more than it's
  useful.
math/matrix: Add transposed_invert() wrappers

+ transposed_invert() is very easy to do in assembly, therefore it's the
  only function for inversing a matrix; however, sometimes we just need
  invert(), so implement it (and non-modifying versions).