Small refactorings as preparation
Fixes for small colour variations in tests

* Most uses of clampTo8Bit actually didn't want 8-bit integers, but just a clamp
  between 0 and 255. There are now two functions. quantizeAndInverse says in its
  comments that these integers are needed, so small colour variations in the
  tests for this cases are expected
* The outputBuffer for quantizeAndInverse was changed back to Int32 to allow
  greater precision in the calculations