Fix Arbitrary instances for color spaces / models with unusual coordinate ranges

[gilgamec]

I noticed that the test specifications for e.g. LAB use the same Abritrary instance as others:

instance (Elevator e, Random e, Illuminant i) => Arbitrary (Color (LAB (i :: k)) e) where
  arbitrary = ColorLAB <$> arbitraryElevator <*> arbitraryElevator <*> arbitraryElevator

where arbitraryElevator gives either the full range of values (for discrete quanta like Word8) or the range [0,1] for Float and Double. However, LAB at least (and possibly others) uses a different range: L can go from 0 to 100, while a and b vary in a similar range, depending on L, and can also be negative. Thus, right now the spec tests are only checking colors very close to black. I'm not sure how this can be handled within QuickCheck.

[lehins]

You are absolutely right about this. We need a new concept of min max values on per channel basis, which has to be at the ColorModel level in order to get this done properly. Arbitrary instance however can be fixed before we get it figure out a proper solution.