Disable softproofing in floating point spaces.

There's a bunch of little niggling bugs that crop up when soft proofing
in floating point spaces, amongst which the inability of the proofing
mechanism to see floating point values higher than 1.0 fall outside of
the gamut of the color space, let alone recognising whether it can go
inside the gamut of the proofing space.

See also http://ninedegreesbelow.com/bug-reports/soft-proofing-problems.html

For now, it's best to disable softproofing on floating point spaces.
Might need a more official check, though.

Ref 2337