[css-masking] Add mask-position-{x,y} #103
Description
The spec should probably add these two properties and make mask-position a shorthand of those two.
WebKit and Blink have implemented -webkit-mask-position-{x,y}, and Gecko is going to ship the unprefixed form mask-position-{x,y} in Firefox 53.
The syntax should be similiar to background-position-{x,y}, so I suppose this isn't something very controversial.