Support flipping mutiple values under a text-shadow or border-shadow rule

[MatmaRex]

CSSJanus only supports flipping one text-shadow or border-shadow, while the CSS syntax allows multiple.

(Originally filed at https://phabricator.wikimedia.org/T62805)

Input:

* {
 text-shadow: red 99px -1px 1px, blue 99px 2px 1px;
}

Expected output:

* {
 text-shadow: red -99px -1px 1px, blue -99px 2px 1px;
}

Actual output:

* {
 text-shadow: red -99px -1px 1px, blue 99px 2px 1px;
}

[YairRand]

(Just noting that #36 also fixes this bug.)

[Krinkle]

Exported to https://phabricator.wikimedia.org/T62805.