Fix maths for lib/functions/adjust()

[buzuck]

The calculation of the returned value is awkward in case of a percentage of lightness.
However, as this error is here since quite a long time, fixing will surely break stuff...

Sorry about the double PR... :/

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Take hsl.l = 60. Increasing it's value of 50% should give as a result 90%, and 30% when decreasing it.
The decreasing part is OK, however increasing the lightness by 50% gives 80%.

The current code, when used with percentages and lightness is equivalent to:

if ( val > 0 ) {
    val = (100 - hsl[prop]) * val / 100
} else {
    val = hsl[prop] * (val / 100);
}
hsl[prop] += val;

Numerically, with values as given above (val = 50, hsl.l = 60):

if ( val > 0 ) {
    val = (100 - 60) * 50 / 100 = 20;
} else {
    val = 60 * (50 / 100) = 30; /* Or = -30 ; as if we are here, val < 0 */
}

Thus, if we want to have 90% as a result, only the else part is valid.

[buzuck]

Mh... It seems this is definitely a breaking change, considering the tests failures.

Here is a dirty fix:

lightness-fix(color, percentage)
	lightness(color, lightness(color) + lightness(color) * percentage / 100)

a = hsl(0, 0%, 60%)

b = a + 50%              /* Legacy behavior: b = hsl(0, 0%, 80%) */
b = lightness-fix(a, 50) /* New behavior: b = hsl(0, 0%, 90%) */

[iChenLei]

Any update ? Bro