Fix desaturation of greyscale colours

[anlutro]

See issue #864

[coveralls]

Coverage decreased (-0.02%) to 76.12% when pulling c8ff945 on alprs:fix_desaturate-grey into 714484e on sass:master.

[QuLogic]

It looks like the related functions also don't clamp results, e.g., saturate does not limit the result to 100. Maybe you could add those fixes too.

[mgreter]

@anlutro Could you come up with a test case which includes all edge cases? If so please add another PR to https://github.com/sass/sass-spec!

[anlutro]

@QuLogic I did that originally but thought it might be misleading because it is originally the saturation of the colour passed to the function.

As for clamping saturate(), it actually seems to be working fine - saturate(red, 50%) still returns red.

saturate() on grey colours behaves weirdly in my opinion, but it behaves weirdly in ruby sass so I suppose it'd be bad form to try and change it in libsass.

I'll see if I can make a PR to the spec.

[xzyfer]

Spec added sass/sass-spec#249

[xzyfer]

@anlutro I've merged your specs which now causes this PR build to fail. Could you please investigate?

https://travis-ci.org/sass/libsass/jobs/48892884

[anlutro]

Looks like I was wrong about saturate() not having the same issue. I can fix that as well I suppose.

[coveralls]

Coverage increased (+0.12%) to 76.21% when pulling 9d2f331 on alprs:fix_desaturate-grey into 4d5ea59 on sass:master.

[QuLogic]

Hmm, this is += now for desaturation?

[anlutro]

Yikes, stupid mistake. Glad you spotted it.

[coveralls]

Coverage increased (+0.12%) to 76.21% when pulling b6b1d2e on alprs:fix_desaturate-grey into 4d5ea59 on sass:master.

[anlutro]

OK, there is one more problem I have to fix - desaturate(saturate(#999, 1%), 10%); should return #999999 but returns #90a2a2.

[anlutro]

Fixed and squashed commits, everything should be OK now.

[coveralls]

Coverage increased (+0.12%) to 76.21% when pulling 657486b on alprs:fix_desaturate-grey into 4d5ea59 on sass:master.

[xzyfer]

Great work @anlutro