[css-masking-1] Add destination-out for mask-composite

[Azmisov]

(A+B)-C mask operation I have found is quite useful (destination-out), but it is the only one of the relevant alpha Porter-Duff composite operations that was not included in the spec. E.g. I want to subtract a mask from both A and B.

• add = source-over/destination-over (commutative)
• subtract = source-out
• intersect = source-in/destination-in (commutative)
• exclude = xor

Note the lack of an equivalent destination-out operation.

I can't think of a way to simulate it with existing operations (add, subtract, intersect, exclude).