fix #4357: -webkit- prefix for mask shorthand

[BPJEnnova]

Added CSS vendor prefix (-webkit) for the mask shorthand property (#4357).

/* Original code */
main {
    mask: url(x.png) center/5rem no-repeat
}

/* Old output */
main {
    mask: url(x.png) center/5rem no-repeat
}

/* New output */
main {
    -webkit-mask: url(x.png) center/5rem no-repeat
    mask: url(x.png) center/5rem no-repeat
}

[evanw]

Looks good! Thanks for the PR.